Re: [PATCH 1/4] perf: Fix system-wide events miscounting during cgroup monitoring
From: Liang, Kan
Date: Mon Apr 29 2019 - 11:29:57 EST
On 4/29/2019 11:04 AM, Mark Rutland wrote:
On Mon, Apr 29, 2019 at 07:44:02AM -0700, kan.liang@xxxxxxxxxxxxxxx wrote:
From: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
When counting system-wide events and cgroup events simultaneously, the
value of system-wide events are miscounting. For example,
perf stat -e cycles,instructions -e cycles,instructions -G
cgroup1,cgroup1,cgroup2,cgroup2 -a -e cycles,instructions -I 1000
1.096265502 12,375,398,872 cycles cgroup1
1.096265502 8,396,184,503 instructions cgroup1
# 0.10 insn per cycle
1.096265502 109,609,027,112 cycles cgroup2
1.096265502 11,533,690,148 instructions cgroup2
# 0.14 insn per cycle
1.096265502 121,672,937,058 cycles
1.096265502 19,331,496,727 instructions #
0.24 insn per cycle
The events are identical events for system-wide and cgroup. The
value of system-wide events is less than the sum of cgroup events,
which is wrong.
Both system-wide and cgroup are per-cpu. They share the same cpuctx
groups, cpuctx->flexible_groups/pinned_groups.
In context switch, cgroup switch tries to schedule all the events in
the cpuctx groups. The unmatched cgroup events can be filtered by its
event->cgrp. However, system-wide events, which event->cgrp is NULL, are
unconditionally switched, which causes miscounting.
Why exactly does that cause mis-counting?
Are the system-wide events not switched back in? Or filtered
erroneously?
The small period between the prev cgroup sched_out and the new cgroup
sched_in is missed for system-wide events.
Current code mistakenly sched_out of system-wide events during that period.
Introduce cgrp_switch in cpuctx to indicate the cgroup context switch.
If it's system-wide event in context switch, don't try to switch it.
Fixes: e5d1367f17ba ("perf: Add cgroup support")
Signed-off-by: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
---
include/linux/perf_event.h | 1 +
kernel/events/core.c | 30 ++++++++++++++++++++++++++++--
2 files changed, 29 insertions(+), 2 deletions(-)
diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index e47ef76..039e2f2 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -795,6 +795,7 @@ struct perf_cpu_context {
#ifdef CONFIG_CGROUP_PERF
struct perf_cgroup *cgrp;
struct list_head cgrp_cpuctx_entry;
+ unsigned int cgrp_switch :1;
#endif
struct list_head sched_cb_entry;
diff --git a/kernel/events/core.c b/kernel/events/core.c
index dc7dead..388dd42 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -809,6 +809,7 @@ static void perf_cgroup_switch(struct task_struct *task, int mode)
perf_ctx_lock(cpuctx, cpuctx->task_ctx);
perf_pmu_disable(cpuctx->ctx.pmu);
+ cpuctx->cgrp_switch = true;
if (mode & PERF_CGROUP_SWOUT) {
cpu_ctx_sched_out(cpuctx, EVENT_ALL);
@@ -832,6 +833,7 @@ static void perf_cgroup_switch(struct task_struct *task, int mode)
&cpuctx->ctx);
cpu_ctx_sched_in(cpuctx, EVENT_ALL, task);
}
+ cpuctx->cgrp_switch = false;
perf_pmu_enable(cpuctx->ctx.pmu);
perf_ctx_unlock(cpuctx, cpuctx->task_ctx);
}
@@ -2944,13 +2946,25 @@ static void ctx_sched_out(struct perf_event_context *ctx,
perf_pmu_disable(ctx->pmu);
if (is_active & EVENT_PINNED) {
- list_for_each_entry_safe(event, tmp, &ctx->pinned_active, active_list)
+ list_for_each_entry_safe(event, tmp, &ctx->pinned_active, active_list) {
+#ifdef CONFIG_CGROUP_PERF
+ /* Don't sched system-wide event when cgroup context switch */
+ if (cpuctx->cgrp_switch && !event->cgrp)
+ continue;
+#endif
This pattern is duplicated several times, and should probably be a
helper like:
static bool skip_cgroup_switch(struct perf_cpu_context *cpuctx,
struct perf_event *event);
{
return IS_ENABLED(CONFIG_CGROUP_PERF) &&
cpuctx->cgrp_switch &&
!event->cgrp;
}
... allowing the above to be an unconditional:
if (skip_cgroup_switch(cpuctx, event))
continue;
... and likewise for the other cases.
I will change it in V2.
Thanks,
Kan
Thanks,
Mark.
group_sched_out(event, cpuctx, ctx);
+ }
}
if (is_active & EVENT_FLEXIBLE) {
- list_for_each_entry_safe(event, tmp, &ctx->flexible_active, active_list)
+ list_for_each_entry_safe(event, tmp, &ctx->flexible_active, active_list) {
+#ifdef CONFIG_CGROUP_PERF
+ /* Don't sched system-wide event when cgroup context switch */
+ if (cpuctx->cgrp_switch && !event->cgrp)
+ continue;
+#endif
group_sched_out(event, cpuctx, ctx);
+ }
}
perf_pmu_enable(ctx->pmu);
}
@@ -3280,6 +3294,12 @@ static int pinned_sched_in(struct perf_event *event, void *data)
if (event->state <= PERF_EVENT_STATE_OFF)
return 0;
+#ifdef CONFIG_CGROUP_PERF
+ /* Don't sched system-wide event when cgroup context switch */
+ if (sid->cpuctx->cgrp_switch && !event->cgrp)
+ return 0;
+#endif
+
if (!event_filter_match(event))
return 0;
@@ -3305,6 +3325,12 @@ static int flexible_sched_in(struct perf_event *event, void *data)
if (event->state <= PERF_EVENT_STATE_OFF)
return 0;
+#ifdef CONFIG_CGROUP_PERF
+ /* Don't sched system-wide event when cgroup context switch */
+ if (sid->cpuctx->cgrp_switch && !event->cgrp)
+ return 0;
+#endif
+
if (!event_filter_match(event))
return 0;
--
2.7.4