Re: [PATCH v4 12/24] x86/resctrl: Make resctrl_arch_rmid_read() retry when it is interrupted

[James Morse <james.morse@arm.com>]

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