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Date: Wed, 12 Sep 2012 20:50:28 +0200 From: Stephane Eranian <eranian@...gle.com> To: Peter Zijlstra <peterz@...radead.org> Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>, Oleg Nesterov <oleg@...hat.com>, linux-kernel <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...nel.org> Subject: Re: [RFC][PATCH] perf, intel: Don't touch MSR_IA32_DEBUGCTLMSR from NMI context On Wed, Sep 12, 2012 at 8:17 PM, Peter Zijlstra <peterz@...radead.org> wrote: > On Wed, 2012-09-12 at 20:00 +0200, Stephane Eranian wrote: >> On Wed, Sep 12, 2012 at 7:45 PM, Peter Zijlstra <peterz@...radead.org> wrote: >> > On Wed, 2012-09-12 at 19:37 +0200, Peter Zijlstra wrote: >> >> Ah, so I do think EIO will re-enable LBR, >> > >> > OK, it does not, but the: >> > >> >> also the handler is wrapped in >> >> x86_pmu::{dis,en}able_all() which does end up calling >> >> intel_pmu_lbr_{dis,en}able_all(). >> > >> > Thing does the re-enable for us, >> > >> >> >> However that leaves the MSR in the >> >> exact same state on exit as it was on enter, so that's not a problem for >> >> the: read-modify-write change. >> > >> > in a safe way. >> Well, I think it does even when we have to stop events (x86_pmu_stop) >> because the buffer is full. Looks like we always re-enable lbr. > > How so, without the proposed patch, the intel_pmu_disable_event() can do > intel_pmu_lbr_disable() which decrements cpuc->lbr_users, so the final > intel_pmu_enable_all()->intel_pmu_lbr_enable_all() will be a NOP, > leaving LBR disabled, where we entered the NMI with LBR enabled. > You're right. I looked at the wrong x86_pmu struct. So yes, it is not symmetrical in all cases. So that's a problem for the race condition. >> So looks like the handler is a wash for debugctl. > > As for BTS, it looks like we don't throttle the thing at all, so we > shouldn't ever get to the asymmetric thing, right? No you do, in the same function: static void intel_pmu_disable_event(struct perf_event *event) { struct hw_perf_event *hwc = &event->hw; struct cpu_hw_events *cpuc = &__get_cpu_var(cpu_hw_events); if (unlikely(hwc->idx == INTEL_PMC_IDX_FIXED_BTS)) { intel_pmu_disable_bts(); intel_pmu_drain_bts_buffer(); return; } -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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