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    SubjectRe: [PATCH 06/14] perf, core: always switch pmu specific data during context switch
    On Wed, Feb 05, 2014 at 06:19:27PM +0100, Stephane Eranian wrote:
    > On Fri, Jan 3, 2014 at 6:48 AM, Yan, Zheng <zheng.z.yan@intel.com> wrote:
    > > If two tasks were both forked from the same parent task, Events in their perf
    > > task contexts can be the same. Perf core optimizes context switch oout in this
    > > case.
    > >
    > > Previous patch inroduces pmu specific data. The data is task specific, so we
    > > should switch the data even when context switch is optimized out.
    > >
    > Reviwed-by: Stephane Eranian <eranian@google.com>

    You should look again.. that xchg() is an atomic op and a total waste of
    time since the assignment back onto ctx->task_ctx_data is non-atomic.

    Complete fail there.

    > > Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
    > > ---
    > > kernel/events/core.c | 2 ++
    > > 1 file changed, 2 insertions(+)
    > >
    > > diff --git a/kernel/events/core.c b/kernel/events/core.c
    > > index b6650ab..d6d8dea 100644
    > > --- a/kernel/events/core.c
    > > +++ b/kernel/events/core.c
    > > @@ -2319,6 +2319,8 @@ static void perf_event_context_sched_out(struct task_struct *task, int ctxn,
    > > next->perf_event_ctxp[ctxn] = ctx;
    > > ctx->task = next;
    > > next_ctx->task = task;
    > > + ctx->task_ctx_data = xchg(&next_ctx->task_ctx_data,
    > > + ctx->task_ctx_data);
    > > do_switch = 0;
    > >
    > > perf_event_sync_stat(ctx, next_ctx);
    > > --
    > > 1.8.4.2
    > >


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