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5aff60a191
An over-committed guest with more vCPUs than pCPUs has a heavy overload in osq_lock(). This is because if vCPU-A holds the osq lock and yields out, vCPU-B ends up waiting for per_cpu node->locked to be set. IOW, vCPU-B waits for vCPU-A to run and unlock the osq lock. Use the new vcpu_is_preempted(cpu) interface to detect if a vCPU is currently running or not, and break out of the spin-loop if so. test case: $ perf record -a perf bench sched messaging -g 400 -p && perf report before patch: 18.09% sched-messaging [kernel.vmlinux] [k] osq_lock 12.28% sched-messaging [kernel.vmlinux] [k] rwsem_spin_on_owner 5.27% sched-messaging [kernel.vmlinux] [k] mutex_unlock 3.89% sched-messaging [kernel.vmlinux] [k] wait_consider_task 3.64% sched-messaging [kernel.vmlinux] [k] _raw_write_lock_irq 3.41% sched-messaging [kernel.vmlinux] [k] mutex_spin_on_owner.is 2.49% sched-messaging [kernel.vmlinux] [k] system_call after patch: 20.68% sched-messaging [kernel.vmlinux] [k] mutex_spin_on_owner 8.45% sched-messaging [kernel.vmlinux] [k] mutex_unlock 4.12% sched-messaging [kernel.vmlinux] [k] system_call 3.01% sched-messaging [kernel.vmlinux] [k] system_call_common 2.83% sched-messaging [kernel.vmlinux] [k] copypage_power7 2.64% sched-messaging [kernel.vmlinux] [k] rwsem_spin_on_owner 2.00% sched-messaging [kernel.vmlinux] [k] osq_lock Suggested-by: Boqun Feng <boqun.feng@gmail.com> Tested-by: Juergen Gross <jgross@suse.com> Signed-off-by: Pan Xinhui <xinhui.pan@linux.vnet.ibm.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Acked-by: Christian Borntraeger <borntraeger@de.ibm.com> Acked-by: Paolo Bonzini <pbonzini@redhat.com> Cc: David.Laight@ACULAB.COM Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: benh@kernel.crashing.org Cc: bsingharora@gmail.com Cc: dave@stgolabs.net Cc: kernellwp@gmail.com Cc: konrad.wilk@oracle.com Cc: linuxppc-dev@lists.ozlabs.org Cc: mpe@ellerman.id.au Cc: paulmck@linux.vnet.ibm.com Cc: paulus@samba.org Cc: rkrcmar@redhat.com Cc: virtualization@lists.linux-foundation.org Cc: will.deacon@arm.com Cc: xen-devel-request@lists.xenproject.org Cc: xen-devel@lists.xenproject.org Link: http://lkml.kernel.org/r/1478077718-37424-3-git-send-email-xinhui.pan@linux.vnet.ibm.com [ Translated to English. ] Signed-off-by: Ingo Molnar <mingo@kernel.org>
218 lines
5.3 KiB
C
218 lines
5.3 KiB
C
#include <linux/percpu.h>
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#include <linux/sched.h>
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#include <linux/osq_lock.h>
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/*
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* An MCS like lock especially tailored for optimistic spinning for sleeping
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* lock implementations (mutex, rwsem, etc).
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*
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* Using a single mcs node per CPU is safe because sleeping locks should not be
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* called from interrupt context and we have preemption disabled while
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* spinning.
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*/
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static DEFINE_PER_CPU_SHARED_ALIGNED(struct optimistic_spin_node, osq_node);
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/*
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* We use the value 0 to represent "no CPU", thus the encoded value
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* will be the CPU number incremented by 1.
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*/
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static inline int encode_cpu(int cpu_nr)
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{
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return cpu_nr + 1;
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}
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static inline int node_cpu(struct optimistic_spin_node *node)
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{
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return node->cpu - 1;
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}
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static inline struct optimistic_spin_node *decode_cpu(int encoded_cpu_val)
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{
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int cpu_nr = encoded_cpu_val - 1;
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return per_cpu_ptr(&osq_node, cpu_nr);
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}
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/*
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* Get a stable @node->next pointer, either for unlock() or unqueue() purposes.
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* Can return NULL in case we were the last queued and we updated @lock instead.
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*/
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static inline struct optimistic_spin_node *
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osq_wait_next(struct optimistic_spin_queue *lock,
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struct optimistic_spin_node *node,
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struct optimistic_spin_node *prev)
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{
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struct optimistic_spin_node *next = NULL;
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int curr = encode_cpu(smp_processor_id());
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int old;
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/*
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* If there is a prev node in queue, then the 'old' value will be
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* the prev node's CPU #, else it's set to OSQ_UNLOCKED_VAL since if
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* we're currently last in queue, then the queue will then become empty.
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*/
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old = prev ? prev->cpu : OSQ_UNLOCKED_VAL;
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for (;;) {
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if (atomic_read(&lock->tail) == curr &&
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atomic_cmpxchg_acquire(&lock->tail, curr, old) == curr) {
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/*
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* We were the last queued, we moved @lock back. @prev
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* will now observe @lock and will complete its
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* unlock()/unqueue().
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*/
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break;
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}
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/*
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* We must xchg() the @node->next value, because if we were to
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* leave it in, a concurrent unlock()/unqueue() from
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* @node->next might complete Step-A and think its @prev is
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* still valid.
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*
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* If the concurrent unlock()/unqueue() wins the race, we'll
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* wait for either @lock to point to us, through its Step-B, or
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* wait for a new @node->next from its Step-C.
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*/
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if (node->next) {
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next = xchg(&node->next, NULL);
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if (next)
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break;
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}
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cpu_relax();
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}
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return next;
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}
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bool osq_lock(struct optimistic_spin_queue *lock)
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{
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struct optimistic_spin_node *node = this_cpu_ptr(&osq_node);
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struct optimistic_spin_node *prev, *next;
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int curr = encode_cpu(smp_processor_id());
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int old;
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node->locked = 0;
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node->next = NULL;
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node->cpu = curr;
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/*
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* We need both ACQUIRE (pairs with corresponding RELEASE in
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* unlock() uncontended, or fastpath) and RELEASE (to publish
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* the node fields we just initialised) semantics when updating
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* the lock tail.
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*/
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old = atomic_xchg(&lock->tail, curr);
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if (old == OSQ_UNLOCKED_VAL)
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return true;
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prev = decode_cpu(old);
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node->prev = prev;
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WRITE_ONCE(prev->next, node);
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/*
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* Normally @prev is untouchable after the above store; because at that
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* moment unlock can proceed and wipe the node element from stack.
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*
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* However, since our nodes are static per-cpu storage, we're
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* guaranteed their existence -- this allows us to apply
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* cmpxchg in an attempt to undo our queueing.
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*/
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while (!READ_ONCE(node->locked)) {
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/*
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* If we need to reschedule bail... so we can block.
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* Use vcpu_is_preempted() to avoid waiting for a preempted
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* lock holder:
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*/
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if (need_resched() || vcpu_is_preempted(node_cpu(node->prev)))
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goto unqueue;
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cpu_relax();
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}
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return true;
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unqueue:
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/*
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* Step - A -- stabilize @prev
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*
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* Undo our @prev->next assignment; this will make @prev's
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* unlock()/unqueue() wait for a next pointer since @lock points to us
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* (or later).
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*/
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for (;;) {
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if (prev->next == node &&
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cmpxchg(&prev->next, node, NULL) == node)
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break;
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/*
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* We can only fail the cmpxchg() racing against an unlock(),
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* in which case we should observe @node->locked becomming
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* true.
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*/
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if (smp_load_acquire(&node->locked))
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return true;
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cpu_relax();
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/*
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* Or we race against a concurrent unqueue()'s step-B, in which
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* case its step-C will write us a new @node->prev pointer.
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*/
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prev = READ_ONCE(node->prev);
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}
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/*
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* Step - B -- stabilize @next
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*
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* Similar to unlock(), wait for @node->next or move @lock from @node
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* back to @prev.
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*/
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next = osq_wait_next(lock, node, prev);
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if (!next)
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return false;
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/*
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* Step - C -- unlink
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*
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* @prev is stable because its still waiting for a new @prev->next
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* pointer, @next is stable because our @node->next pointer is NULL and
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* it will wait in Step-A.
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*/
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WRITE_ONCE(next->prev, prev);
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WRITE_ONCE(prev->next, next);
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return false;
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}
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void osq_unlock(struct optimistic_spin_queue *lock)
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{
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struct optimistic_spin_node *node, *next;
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int curr = encode_cpu(smp_processor_id());
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/*
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* Fast path for the uncontended case.
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*/
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if (likely(atomic_cmpxchg_release(&lock->tail, curr,
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OSQ_UNLOCKED_VAL) == curr))
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return;
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/*
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* Second most likely case.
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*/
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node = this_cpu_ptr(&osq_node);
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next = xchg(&node->next, NULL);
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if (next) {
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WRITE_ONCE(next->locked, 1);
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return;
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}
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next = osq_wait_next(lock, node, NULL);
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if (next)
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WRITE_ONCE(next->locked, 1);
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}
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