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author | Peter Zijlstra <peterz@infradead.org> | 2016-03-17 14:17:35 (GMT) |
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committer | Ingo Molnar <mingo@kernel.org> | 2016-03-21 08:08:17 (GMT) |
commit | 91a612eea9a316c464cc170ff8492ec09e7d1c69 (patch) | |
tree | 206ff774df73c92cf24196aca96e6eb325aaa9b6 | |
parent | c2872d381f1ab1bc39c7f044644bf1d3da7c11dd (diff) | |
download | linux-91a612eea9a316c464cc170ff8492ec09e7d1c69.tar.xz |
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>
-rw-r--r-- | kernel/events/core.c | 87 |
1 files changed, 51 insertions, 36 deletions
diff --git a/kernel/events/core.c b/kernel/events/core.c index d394773..43e35fa 100644 --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -376,8 +376,11 @@ static void update_perf_cpu_limits(void) u64 tmp = perf_sample_period_ns; tmp *= sysctl_perf_cpu_time_max_percent; - do_div(tmp, 100); - ACCESS_ONCE(perf_sample_allowed_ns) = tmp; + tmp = div_u64(tmp, 100); + if (!tmp) + tmp = 1; + + WRITE_ONCE(perf_sample_allowed_ns, tmp); } static int perf_rotate_context(struct perf_cpu_context *cpuctx); @@ -409,7 +412,13 @@ int perf_cpu_time_max_percent_handler(struct ctl_table *table, int write, if (ret || !write) return ret; - update_perf_cpu_limits(); + if (sysctl_perf_cpu_time_max_percent == 100) { + printk(KERN_WARNING + "perf: Dynamic interrupt throttling disabled, can hang your system!\n"); + WRITE_ONCE(perf_sample_allowed_ns, 0); + } else { + update_perf_cpu_limits(); + } return 0; } @@ -423,62 +432,68 @@ int perf_cpu_time_max_percent_handler(struct ctl_table *table, int write, #define NR_ACCUMULATED_SAMPLES 128 static DEFINE_PER_CPU(u64, running_sample_length); +static u64 __report_avg; +static u64 __report_allowed; + static void perf_duration_warn(struct irq_work *w) { - u64 allowed_ns = ACCESS_ONCE(perf_sample_allowed_ns); - u64 avg_local_sample_len; - u64 local_samples_len; - - local_samples_len = __this_cpu_read(running_sample_length); - avg_local_sample_len = local_samples_len/NR_ACCUMULATED_SAMPLES; - printk_ratelimited(KERN_WARNING - "perf interrupt took too long (%lld > %lld), lowering " - "kernel.perf_event_max_sample_rate to %d\n", - avg_local_sample_len, allowed_ns >> 1, - sysctl_perf_event_sample_rate); + "perf: interrupt took too long (%lld > %lld), lowering " + "kernel.perf_event_max_sample_rate to %d\n", + __report_avg, __report_allowed, + sysctl_perf_event_sample_rate); } static DEFINE_IRQ_WORK(perf_duration_work, perf_duration_warn); void perf_sample_event_took(u64 sample_len_ns) { - u64 allowed_ns = ACCESS_ONCE(perf_sample_allowed_ns); - u64 avg_local_sample_len; - u64 local_samples_len; + u64 max_len = READ_ONCE(perf_sample_allowed_ns); + u64 running_len; + u64 avg_len; + u32 max; - if (allowed_ns == 0) + if (max_len == 0) return; - /* decay the counter by 1 average sample */ - local_samples_len = __this_cpu_read(running_sample_length); - local_samples_len -= local_samples_len/NR_ACCUMULATED_SAMPLES; - local_samples_len += sample_len_ns; - __this_cpu_write(running_sample_length, local_samples_len); + /* Decay the counter by 1 average sample. */ + running_len = __this_cpu_read(running_sample_length); + running_len -= running_len/NR_ACCUMULATED_SAMPLES; + running_len += sample_len_ns; + __this_cpu_write(running_sample_length, running_len); /* - * note: this will be biased artifically low until we have - * seen NR_ACCUMULATED_SAMPLES. Doing it this way keeps us + * Note: this will be biased artifically low until we have + * seen NR_ACCUMULATED_SAMPLES. Doing it this way keeps us * from having to maintain a count. */ - avg_local_sample_len = local_samples_len/NR_ACCUMULATED_SAMPLES; - - if (avg_local_sample_len <= allowed_ns) + avg_len = running_len/NR_ACCUMULATED_SAMPLES; + if (avg_len <= max_len) return; - if (max_samples_per_tick <= 1) - return; + __report_avg = avg_len; + __report_allowed = max_len; - max_samples_per_tick = DIV_ROUND_UP(max_samples_per_tick, 2); - sysctl_perf_event_sample_rate = max_samples_per_tick * HZ; - perf_sample_period_ns = NSEC_PER_SEC / sysctl_perf_event_sample_rate; + /* + * Compute a throttle threshold 25% below the current duration. + */ + avg_len += avg_len / 4; + max = (TICK_NSEC / 100) * sysctl_perf_cpu_time_max_percent; + if (avg_len < max) + max /= (u32)avg_len; + else + max = 1; - update_perf_cpu_limits(); + WRITE_ONCE(perf_sample_allowed_ns, avg_len); + WRITE_ONCE(max_samples_per_tick, max); + + sysctl_perf_event_sample_rate = max * HZ; + perf_sample_period_ns = NSEC_PER_SEC / sysctl_perf_event_sample_rate; if (!irq_work_queue(&perf_duration_work)) { - early_printk("perf interrupt took too long (%lld > %lld), lowering " + early_printk("perf: interrupt took too long (%lld > %lld), lowering " "kernel.perf_event_max_sample_rate to %d\n", - avg_local_sample_len, allowed_ns >> 1, + __report_avg, __report_allowed, sysctl_perf_event_sample_rate); } } |