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 <[email protected]> > --- > 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.
