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perf/core: Fix dynamic interrupt throttle
There were two problems with the dynamic interrupt throttle mechanism, both triggered by the same action. When you (or perf_fuzzer) write a huge value into /proc/sys/kernel/perf_event_max_sample_rate the computed perf_sample_allowed_ns becomes 0. This effectively disables the whole dynamic throttle. This is fixed by ensuring update_perf_cpu_limits() never sets the value to 0. However, we allow disabling of the dynamic throttle by writing 100 to /proc/sys/kernel/perf_cpu_time_max_percent. This will generate a warning in dmesg. The second problem is that by setting the max_sample_rate to a huge number, the adaptive process can take a few tries, since it halfs the limit each time. Change that to directly compute a new value based on the observed duration. Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com> Cc: Andy Lutomirski <luto@amacapital.net> Cc: Arnaldo Carvalho de Melo <acme@redhat.com> Cc: Borislav Petkov <bp@alien8.de> Cc: Brian Gerst <brgerst@gmail.com> Cc: David Ahern <dsahern@gmail.com> Cc: Denys Vlasenko <dvlasenk@redhat.com> Cc: H. Peter Anvin <hpa@zytor.com> Cc: Jiri Olsa <jolsa@redhat.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Namhyung Kim <namhyung@kernel.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Stephane Eranian <eranian@google.com> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Vince Weaver <vincent.weaver@maine.edu> Signed-off-by: Ingo Molnar <mingo@kernel.org>
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@ -376,8 +376,11 @@ static void update_perf_cpu_limits(void)
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u64 tmp = perf_sample_period_ns;
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tmp *= sysctl_perf_cpu_time_max_percent;
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do_div(tmp, 100);
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ACCESS_ONCE(perf_sample_allowed_ns) = tmp;
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tmp = div_u64(tmp, 100);
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if (!tmp)
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tmp = 1;
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WRITE_ONCE(perf_sample_allowed_ns, tmp);
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}
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static int perf_rotate_context(struct perf_cpu_context *cpuctx);
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@ -409,7 +412,13 @@ int perf_cpu_time_max_percent_handler(struct ctl_table *table, int write,
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if (ret || !write)
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return ret;
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update_perf_cpu_limits();
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if (sysctl_perf_cpu_time_max_percent == 100) {
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printk(KERN_WARNING
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"perf: Dynamic interrupt throttling disabled, can hang your system!\n");
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WRITE_ONCE(perf_sample_allowed_ns, 0);
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} else {
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update_perf_cpu_limits();
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}
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return 0;
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}
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@ -423,62 +432,68 @@ int perf_cpu_time_max_percent_handler(struct ctl_table *table, int write,
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#define NR_ACCUMULATED_SAMPLES 128
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static DEFINE_PER_CPU(u64, running_sample_length);
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static u64 __report_avg;
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static u64 __report_allowed;
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static void perf_duration_warn(struct irq_work *w)
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{
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u64 allowed_ns = ACCESS_ONCE(perf_sample_allowed_ns);
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u64 avg_local_sample_len;
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u64 local_samples_len;
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local_samples_len = __this_cpu_read(running_sample_length);
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avg_local_sample_len = local_samples_len/NR_ACCUMULATED_SAMPLES;
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printk_ratelimited(KERN_WARNING
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"perf interrupt took too long (%lld > %lld), lowering "
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"kernel.perf_event_max_sample_rate to %d\n",
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avg_local_sample_len, allowed_ns >> 1,
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sysctl_perf_event_sample_rate);
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"perf: interrupt took too long (%lld > %lld), lowering "
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"kernel.perf_event_max_sample_rate to %d\n",
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__report_avg, __report_allowed,
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sysctl_perf_event_sample_rate);
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}
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static DEFINE_IRQ_WORK(perf_duration_work, perf_duration_warn);
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void perf_sample_event_took(u64 sample_len_ns)
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{
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u64 allowed_ns = ACCESS_ONCE(perf_sample_allowed_ns);
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u64 avg_local_sample_len;
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u64 local_samples_len;
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u64 max_len = READ_ONCE(perf_sample_allowed_ns);
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u64 running_len;
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u64 avg_len;
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u32 max;
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if (allowed_ns == 0)
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if (max_len == 0)
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return;
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/* decay the counter by 1 average sample */
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local_samples_len = __this_cpu_read(running_sample_length);
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local_samples_len -= local_samples_len/NR_ACCUMULATED_SAMPLES;
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local_samples_len += sample_len_ns;
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__this_cpu_write(running_sample_length, local_samples_len);
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/* Decay the counter by 1 average sample. */
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running_len = __this_cpu_read(running_sample_length);
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running_len -= running_len/NR_ACCUMULATED_SAMPLES;
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running_len += sample_len_ns;
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__this_cpu_write(running_sample_length, running_len);
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/*
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* note: this will be biased artifically low until we have
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* seen NR_ACCUMULATED_SAMPLES. Doing it this way keeps us
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* Note: this will be biased artifically low until we have
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* seen NR_ACCUMULATED_SAMPLES. Doing it this way keeps us
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* from having to maintain a count.
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*/
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avg_local_sample_len = local_samples_len/NR_ACCUMULATED_SAMPLES;
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if (avg_local_sample_len <= allowed_ns)
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avg_len = running_len/NR_ACCUMULATED_SAMPLES;
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if (avg_len <= max_len)
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return;
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if (max_samples_per_tick <= 1)
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return;
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__report_avg = avg_len;
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__report_allowed = max_len;
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max_samples_per_tick = DIV_ROUND_UP(max_samples_per_tick, 2);
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sysctl_perf_event_sample_rate = max_samples_per_tick * HZ;
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/*
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* Compute a throttle threshold 25% below the current duration.
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*/
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avg_len += avg_len / 4;
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max = (TICK_NSEC / 100) * sysctl_perf_cpu_time_max_percent;
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if (avg_len < max)
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max /= (u32)avg_len;
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else
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max = 1;
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WRITE_ONCE(perf_sample_allowed_ns, avg_len);
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WRITE_ONCE(max_samples_per_tick, max);
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sysctl_perf_event_sample_rate = max * HZ;
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perf_sample_period_ns = NSEC_PER_SEC / sysctl_perf_event_sample_rate;
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update_perf_cpu_limits();
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if (!irq_work_queue(&perf_duration_work)) {
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early_printk("perf interrupt took too long (%lld > %lld), lowering "
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early_printk("perf: interrupt took too long (%lld > %lld), lowering "
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"kernel.perf_event_max_sample_rate to %d\n",
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avg_local_sample_len, allowed_ns >> 1,
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__report_avg, __report_allowed,
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sysctl_perf_event_sample_rate);
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}
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}
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