Messages in this thread | | | Date | Fri, 3 Apr 2009 13:32:19 +0200 | From | Ingo Molnar <> | Subject | Re: [patch 04/18] x86, bts: wait until traced task has been scheduled out |
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* Metzger, Markus T <markus.t.metzger@intel.com> wrote:
> >-----Original Message----- > >From: Ingo Molnar [mailto:mingo@elte.hu] > >Sent: Thursday, April 02, 2009 9:18 PM > >To: Metzger, Markus T > >Cc: tglx@linutronix.de; hpa@zytor.com; markus.t.metzger@gmail.com; roland@redhat.com; > >eranian@googlemail.com; oleg@redhat.com; Villacis, Juan; ak@linux.jf.intel.com; linux- > >kernel@vger.kernel.org > >Subject: Re: [patch 04/18] x86, bts: wait until traced task has been scheduled out > > > > > >* markus.t.metzger@intel.com <markus.t.metzger@intel.com> wrote: > > > >> In order to stop branch tracing for a running task, we need to > >> first clear the branch tracing control bits before we may free the > >> tracing buffer. If the traced task is running, the cpu might still > >> trace that task after the branch trace control bits have cleared. > >> > >> Wait until the traced task has been scheduled out before > >> proceeding. > >> > >> A similar problem affects the task debug store context. We first > >> remove the context, then we need to wait until the task has been > >> scheduled out before we can free the context memory. > >> > >> > >> Reviewed-by: Oleg Nesterov <oleg@redhat.com> > >> Signed-off-by: Markus Metzger <markus.t.metzger@intel.com> > >> --- > >> arch/x86/kernel/ds.c | 40 40 + 0 - 0 ! > >> 1 file changed, 40 insertions(+) > >> > >> Index: b/arch/x86/kernel/ds.c > >> =================================================================== > >> --- a/arch/x86/kernel/ds.c > >> +++ b/arch/x86/kernel/ds.c > >> @@ -250,6 +250,40 @@ static DEFINE_PER_CPU(struct ds_context > >> #define system_context per_cpu(system_context_array, smp_processor_id()) > >> > >> > >> +/* > >> + * Wait for the traced task to unschedule. > >> + * > >> + * This guarantees that the bts trace configuration has been > >> + * synchronized with the cpu executing the task. > >> + */ > >> +static void wait_to_unschedule(struct task_struct *task) > >> +{ > > > >this should be in sched.c and task_is_running() should not be > >exported from there. > > > >I.e. your original patch which i objected to is probably the right > >one, but should be named something like "task_wait_context_switch()" > >- which signals its purpose: that it is to wait for the task to > >context-switch at least once, so that its ptrace state is installed > >(or deinstalled) for sure. > > OK. > > I'll move it to sched.c. > > In that case, I would use task_running() without holding the rq > lock, since we don't really care whether we read an old value or > not. Would that be OK with you?
i'd have to see that in full context - reading non-locked results can in essence result in stale old values being read out, regardless of current reality. task_running() is normally used within the rq lock.
Ingo
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