Re: [PATCH v1] perf/core: fix restoring of Intel LBR call stack on a context switch

[Alexey Budankov]

On 15.10.2019 12:16, Peter Zijlstra wrote:
> On Mon, Oct 14, 2019 at 09:08:34AM +0300, Alexey Budankov wrote:
>>
>> Restore Intel LBR call stack from cloned inactive task perf context on
>> a context switch. This change inherently addresses inconsistency in LBR 
>> call stack data provided on a sample in record profiling mode for 
>> example like this:
>>
>>   $ perf record -N -B -T -R --call-graph lbr \
>>          -e cpu/period=0xcdfe60,event=0x3c,name=\'CPU_CLK_UNHALTED.THREAD\'/Duk \
>>          --clockid=monotonic_raw -- ./miniFE.x nx 25 ny 25 nz 25
>>
>> Let's assume threads A, B, C belonging to the same process. 
>> B and C are siblings of A and their perf contexts are treated as equivalent.
>> At some point B blocks on a futex (non preempt context switch).
>> B's LBRs are preserved at B's perf context task_ctx_data and B's events 
>> are removed from PMU and disabled. B's perf context becomes inactive.
>>
>> Later C gets on a cpu, runs, gets profiled and eventually switches to 
>> the awaken but not yet running B. The optimized context switch path is 
>> executed coping B's task_ctx_data to C's one and updating B's perf context 
>> pointer to refer to C's task_ctx_data that contains preserved B's LBRs 
>> after coping.
>>
>> However, as far B's perf context is inactive there is no enabled events
>> in there and B's task_ctx_data->lbr_callstack_users is equal to 0.
>> When B gets on the cpu B's events reviving is skipped following
>> the optimized context switch path and B's task_ctx_data->lbr_callstack_users
>> remains 0. Thus B's LBR's are not restored by pmu sched_task() code called 
>> in the end of perf context switch sched_in callback for B.
>>
>> In the report that manifests as having short fragments of B's
>> call stack, still tracked by LBR's HW between adjacent samples,
>> but the whole thread call tree doesn't aggregate.
>>
> 
>> Signed-off-by: Alexey Budankov <alexey.budankov@xxxxxxxxxxxxxxx>
>> ---
>>  kernel/events/core.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/kernel/events/core.c b/kernel/events/core.c
>> index 2aad959e6def..74c2ff38e079 100644
>> --- a/kernel/events/core.c
>> +++ b/kernel/events/core.c
>> @@ -3181,7 +3181,7 @@ static void perf_event_context_sched_out(struct task_struct *task, int ctxn,
>>  
>>  	rcu_read_lock();
>>  	next_ctx = next->perf_event_ctxp[ctxn];
>> -	if (!next_ctx)
>> +	if (!next_ctx || !next_ctx->is_active)
>>  		goto unlock;
> 
> AFAICT this completely kills off the optimization. next_ctx->is_active
> cannot be set at this point.

Hmm, the intention was to skip optimization path only once when switching 
to just resumed thread. Thanks for observation.

~Alexey