Messages in this thread | | | Date | Mon, 24 Jun 2013 09:46:56 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH v2] perf,x86: Fix shared register mutual exclusion enforcement |
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On Thu, Jun 20, 2013 at 06:42:54PM +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. As an example on Intel > Nehalem, consider the following events: > > group1= L1D_CACHE_LD:E_STATE,OFFCORE_RESPONSE_0:PF_RFO,L1D_CACHE_LD:I_STATE > group2= L1D_CACHE_LD:I_STATE > > The L1D_CACHE_LD event can only be measured by 2 counters. Yet, there > are 3 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 3rd instance of L1D_CACHE_LD). > 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. In other words, there is no guarantee on what is > measured. > > 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. > > Signed-off-by: Stephane Eranian <eranian@google.com>
OK, so I 'accidentally' read the patch again; and noticed something.
In your case above; the get/put constraints are still fully matched. That is; the first group, which was successful, will not have done a put yet. So a subsequent get+put should still leave us with a positive 'ref' count and not undo things.
Only once these events pass through x86_pmu_del() will they get a final put and the 'ref' count will drop to 0.
Now the problem seems to be the get/put things don't actually count properly.
However, if we look at __intel_shared_reg_{get,put}_constraints() there is a refcount in there; namely era->ref; however we don't appear to clear reg->alloc based on it.
Should we?
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