Re: ✗ Fi.CI.IGT: failure for drm/i915: Use seqlock in engine stats (rev2)

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 16/02/2018 10:13, Chris Wilson wrote:
> Quoting Patchwork (2018-02-15 23:50:52)
>> == Series Details ==
>>
>> Series: drm/i915: Use seqlock in engine stats (rev2)
>> URL   : https://patchwork.freedesktop.org/series/38347/
>> State : failure
>>
>> == Summary ==
>>
>> Test perf_pmu:
>>          Subgroup busy-start-vcs0:
>>                  pass       -> DMESG-WARN (shard-apl)
>>          Subgroup busy-idle-check-all-vecs0:
>>                  pass       -> DMESG-WARN (shard-apl)
>>          Subgroup render-node-busy-idle-vcs0:
>>                  pass       -> DMESG-WARN (shard-apl)
>>          Subgroup most-busy-idle-check-all-bcs0:
>>                  pass       -> DMESG-WARN (shard-apl)
>>          Subgroup busy-idle-check-all-bcs0:
>>                  pass       -> DMESG-WARN (shard-apl)
>>          Subgroup most-busy-check-all-bcs0:
>>                  pass       -> DMESG-WARN (shard-apl)
>>          Subgroup render-node-busy-bcs0:
>>                  pass       -> DMESG-WARN (shard-apl)
>>          Subgroup busy-idle-vecs0:
>>                  pass       -> DMESG-WARN (shard-apl)
> 
> Afaict, it's complaining that because the writer is special the reader
> needs irq protection. Which I don't understand, as the point is that the
> writer can run concurrently to the readers (the readers have to
> restart). What am I missing?

As discussed on IRC, issue is courtesy of perf read callback running in 
hardirq context, that the reader can interrupt the writer and so 
incorrectly declare a stable sequence number while reading the mixed up 
version of the underlying data.

Writer:
   seqno++
   modify some fields
   <Reader IRQ...
     read seqno
     read fields
     re-read seqno -> OK
   >..Reader IRQ
   modify other fields
   seqno++

So we need to go back to the first, irqsave version. Do you want to 
pursue this change right now or leave it for some future work?

Regards,

Tvrtko
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