Messages in this thread Patch in this message | | | Date | Tue, 6 Jun 2023 18:03:43 +0100 | Subject | Re: [PATCH v4 12/24] x86/resctrl: Make resctrl_arch_rmid_read() retry when it is interrupted | From | James Morse <> |
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Hi Peter,
On 06/06/2023 09:49, Peter Newman wrote: > On Thu, May 25, 2023 at 8:03 PM James Morse <james.morse@arm.com> wrote: >> +interrupted: >> + am = get_arch_mbm_state(hw_dom, rmid, eventid); >> + if (am) >> + start_msr_val = atomic64_read(&am->prev_msr); >> + >> ret = __rmid_read(rmid, eventid, &msr_val); >> if (ret) >> return ret; >> >> am = get_arch_mbm_state(hw_dom, rmid, eventid); >> if (am) { >> - am->chunks += mbm_overflow_count(am->prev_msr, msr_val, >> - hw_res->mbm_width); >> - chunks = get_corrected_mbm_count(rmid, am->chunks); >> - am->prev_msr = msr_val; >> + old_msr_val = atomic64_cmpxchg(&am->prev_msr, start_msr_val, >> + msr_val); >> + if (old_msr_val != start_msr_val) >> + goto interrupted; >> + >> + chunks = mbm_overflow_count(start_msr_val, msr_val, >> + hw_res->mbm_width); >> + atomic64_add(chunks, &am->chunks); >> + >> + chunks = get_corrected_mbm_count(rmid, >> + atomic64_read(&am->chunks)); >> } else { >> chunks = msr_val; >> } > > It looks like if __rmid_read() is interrupted by an occupancy counter > read between writing QM_EVTSEL and reading QM_CTR, it will not perform > any update to am->prev_msr, and the interrupted read will return the > same counter value as in the interrupting read.
Yup, that's a problem. I was only looking at the mbm state in memory, not the CPU register. I think the fix is to read back QM_EVTSEL after reading QM_CTR. I'll do this in __rmid_read() to avoid returning -EINTR. It creates two retry loops which is annoying, but making the window larger means you're more likely to see false positives.
----------------------------%<---------------------------- diff --git a/arch/x86/kernel/cpu/resctrl/monitor.c b/arch/x86/kernel/cpu/resctrl /monitor.c index e24390d2e661..aeba035bb680 100644 --- a/arch/x86/kernel/cpu/resctrl/monitor.c +++ b/arch/x86/kernel/cpu/resctrl/monitor.c @@ -101,6 +101,7 @@ static inline u64 get_corrected_mbm_count(u32 rmid, unsigned long val)
static int __rmid_read(u32 rmid, enum resctrl_event_id eventid, u64 *val) { + u32 _rmid, _eventid; u64 msr_val;
/* @@ -110,9 +111,15 @@ static int __rmid_read(u32 rmid, enum resctrl_event_id eventid, u64 *val) * IA32_QM_CTR.data (bits 61:0) reports the monitored data. * IA32_QM_CTR.Error (bit 63) and IA32_QM_CTR.Unavailable (bit 62) * are error bits. + * QM_EVTSEL is re-read to detect if this function was interrupted by + * another call, meaning the QM_CTR value may belong to a different + * event. */ - wrmsr(MSR_IA32_QM_EVTSEL, eventid, rmid); - rdmsrl(MSR_IA32_QM_CTR, msr_val); + do { + wrmsr(MSR_IA32_QM_EVTSEL, eventid, rmid); + rdmsrl(MSR_IA32_QM_CTR, msr_val); + rdmsr(MSR_IA32_QM_EVTSEL, _eventid, _rmid); + } while (eventid != _eventid || rmid != _rmid);
if (msr_val & RMID_VAL_ERROR) return -EIO; ----------------------------%<----------------------------
Thanks!
James
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