Messages in this thread | | | Date | Thu, 20 Jun 2013 14:07:43 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH] perf,x86: Fix shared registers mutual exclusion bug |
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* Stephane Eranian <eranian@google.com> wrote:
> On Tue, Jun 18, 2013 at 11:11 AM, Peter Zijlstra <peterz@infradead.org> wrote: > > On Wed, Jun 05, 2013 at 04:43:46PM +0200, Stephane Eranian wrote: > >> > >> This patch fixes a problem with the shared registers mutual > >> exclusion code and incremental event scheduling by the > >> generic perf_event code. > >> > >> There was a bug whereby the mutual exclusion on the shared > >> registers was not enforced because of incremental scheduling > >> abort due to event constraints. > >> > >> Example on Nehalem: > >> group1= ref-cycles,OFFCORE_RESPONSE_0:PF_RFO > >> group2= ref-cycles > >> > >> The ref-cycles event can only be measured by 1 counter. Yet, there > >> are 2 instances here. The first group can be scheduled and is committed. > >> Then, the generic code tries to schedule group2 and this fails (because > >> there is no more counter to support the 2nd instance of ref-cycles). > >> > >> But in x86_schedule_events() error path, put_event_contraints() is invoked > >> on ALL the events and not just the ones that just failed. That causes the > >> "lock" on the shared offcore_response MSR to be released. Yet the first group > >> is actually scheduled and is exposed to reprogramming of that shared msr by > >> the sibling HT thread (when they are shared by HT threads). In other words, > >> there is no guarantee on what is measured for the offcore_response event. > >> > >> This patch fixes the problem by tagging committed events with the > >> PERF_X86_EVENT_COMMITTED tag. In the error path of x86_schedule_events(), > >> only the events NOT tagged have their constraint released. The tag > >> is eventually removed when the event in descheduled. > >> > >> Example was given with offcore_response but also applies to LBR_SELECT > >> and LDLAT shared registers. > >> > >> Signed-off-by: Stephane Eranian <eranian@google.com> > > > > I'm getting conflicts against other patches -- most notably I think the > > contraints stack opt from Andrew Hunter. > > > Yes, that would not surprise me. I wrote this patch without assuming > Andrew's patch would be there. But we need to add it. Then we can fix > the shared_regs patch. > > > I'll try and get Ingo to finally pick up my queued patches so we can > > rebase. > > Ok, thanks.
That happened yesterday, so latest -tip should be a good base to work on.
Thanks,
Ingo
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