Messages in this thread | | | Subject | Re: [PATCH 4/5] perf_events: add cgroup support (v7) | From | Peter Zijlstra <> | Date | Thu, 06 Jan 2011 11:15:09 +0100 |
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On Wed, 2011-01-05 at 22:39 +0100, Stephane Eranian wrote: > Peter, > > On Wed, Jan 5, 2011 at 2:01 PM, Stephane Eranian <eranian@google.com> wrote: > > On Wed, Jan 5, 2011 at 12:23 PM, Peter Zijlstra <peterz@infradead.org> wrote: > >> On Mon, 2011-01-03 at 18:20 +0200, Stephane Eranian wrote: > >>> +void > >>> +perf_cgroup_switch(struct task_struct *task, struct task_struct *next) > >>> +{ > >>> + struct perf_cgroup *cgrp_out = perf_cgroup_from_task(task); > >>> + struct perf_cgroup *cgrp_in = perf_cgroup_from_task(next); > >>> + struct perf_cpu_context *cpuctx; > >>> + struct pmu *pmu; > >>> + /* > >>> + * if task is DEAD, then css_out is irrelevant, it has > >>> + * been changed to init_cgrp in cgroup_exit() from do_exit(). > >>> + * Furthermore, perf_cgroup_exit_task(), has scheduled out > >>> + * all css constrained events, only unconstrained events > >>> + * remain. Therefore we need to reschedule based on css_in. > >>> + */ > >>> + if (task->state != TASK_DEAD && cgrp_out == cgrp_in) > >>> + return; > >> > >> I think that check is broken, TASK_DEAD is set way after calling > >> cgroup_exit(), so if we get preempted in between there you'll still go > >> funny. > >> > > I looked at this part again. > > The original code checking for TASK_DEAD is correct. > > The reason is simple, you're looking at perf_cgroup_switch() which is > invoked as part of schedule() and NOT perf_event_task_exit() (called > prior to cgroup_exit()). > Thus, by the time you do the final schedule(), the task state has indeed > been switched to TASK_DEAD. > > I remember testing for this condition during the debug phase.
But, cgroup_exit() detaches the task from the cgroup, after which the cgroup can disappear. Furthermore, we can schedule after cgroup_exit() and before the explicit schedule() invocation.
Some of the exit functions (say proc_exit_connector) can block and cause scheduling, and with PREEMPT=y we can get preempted.
This means you'll be context switching, and thus possibly calling perf_cgroup_switch(), on a task who's cgroup is possibly destroyed.
So I'm not at all seeing how this is correct.
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