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Peter Anvin" , linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@meta.com, stable@vger.kernel.org References: <20260310-perf-v2-1-4a3156fce43c@debian.org> <20260311173509.GR606826@noisy.programming.kicks-ass.net> <20260311204035.GX606826@noisy.programming.kicks-ass.net> Content-Language: en-US From: "Mi, Dapeng" In-Reply-To: <20260311204035.GX606826@noisy.programming.kicks-ass.net> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 3/12/2026 4:40 AM, Peter Zijlstra wrote: > On Wed, Mar 11, 2026 at 06:35:09PM +0100, Peter Zijlstra wrote: >>> Additionally, does this change leave the unthrottled event's hardware >>> counter uninitialized? >> Also yes. > Something like so on top of things I suppose. > > --- > Subject: x86/perf: Make sure to program the counter value for stopped events on migration > From: Peter Zijlstra > Date: Wed Mar 11 21:29:14 CET 2026 > > Both Mi Dapeng and Ian Rogers noted that not everything that sets HES_STOPPED > is required to EF_UPDATE. Specifically the 'step 1' loop of rescheduling > explicitly does EF_UPDATE to ensure the counter value is read. > > However, then 'step 2' simply leaves the new counter uninitialized when > HES_STOPPED, even though, as noted above, the thing that stopped them might not > be aware it needs to EF_RELOAD -- since it didn't EF_UPDATE on stop. > > One such location that is affected is throttling, throttle does pmu->stop(, 0); > and unthrottle does pmu->start(, 0); possibly restarting an uninitialized counter. > > Fixes: a4eaf7f14675 ("perf: Rework the PMU methods") > Reported-by: Dapeng Mi > Reported-by: Ian Rogers > Signed-off-by: Peter Zijlstra (Intel) > --- > arch/x86/events/core.c | 4 +++- > 1 file changed, 3 insertions(+), 1 deletion(-) > > --- a/arch/x86/events/core.c > +++ b/arch/x86/events/core.c > @@ -1374,8 +1374,10 @@ static void x86_pmu_enable(struct pmu *p > > cpuc->events[hwc->idx] = event; > > - if (hwc->state & PERF_HES_ARCH) > + if (hwc->state & PERF_HES_ARCH) { > + static_call(x86_pmu_set_period)(event); > continue; > + } > > /* > * if cpuc->enabled = 0, then no wrmsr as LGTM. Reviewed-by: Dapeng Mi >