Messages in this thread |  | | Date | Mon, 28 Aug 2017 16:12:01 -0700 | From | Vikram Mulukutla <> | Subject | Re: [Question]: try to fix contention between expire_timers and try_to_del_timer_sync |
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Hi Will,
On 2017-08-25 12:48, Vikram Mulukutla wrote: > Hi Will, > > On 2017-08-15 11:40, Will Deacon wrote: >> Hi Vikram, >> >> On Thu, Aug 03, 2017 at 04:25:12PM -0700, Vikram Mulukutla wrote: >>> On 2017-07-31 06:13, Will Deacon wrote: >>> >On Fri, Jul 28, 2017 at 12:09:38PM -0700, Vikram Mulukutla wrote: >>> >>On 2017-07-28 02:28, Will Deacon wrote: >>> >>>On Thu, Jul 27, 2017 at 06:10:34PM -0700, Vikram Mulukutla wrote: >>> >>> >>> >>> >>This does seem to help. Here's some data after 5 runs with and without >>> >>the >>> >>patch. >>> > >>> >Blimey, that does seem to make a difference. Shame it's so ugly! Would you >>> >be able to experiment with other values for CPU_RELAX_WFE_THRESHOLD? I had >>> >it set to 10000 in the diff I posted, but that might be higher than >>> >optimal. >>> >It would be interested to see if it correlates with num_possible_cpus() >>> >for the highly contended case. >>> > >>> >Will >>> >>> Sorry for the late response - I should hopefully have some more data >>> with >>> different thresholds before the week is finished or on Monday. >> >> Did you get anywhere with the threshold heuristic? >> >> Will > > Here's some data from experiments that I finally got to today. I > decided > to recompile for every value of the threshold. Was doing a binary > search > of sorts and then started reducing by orders of magnitude. There pairs > of rows here: >
Well here's something interesting. I tried a different platform and found that the workaround doesn't help much at all, similar to Qiao's observation on his b.L chipset. Something to do with the WFE implementation or event-stream?
I modified your patch to use a __delay(1) in place of the WFEs and this was the result (still with the 10k threshold). The worst-case lock time for cpu0 drastically improves. Given that cpu0 re-enables interrupts between each lock attempt in my test case, I think the lock count matters less here.
cpu_relax() patch with WFEs (original workaround): (pairs of rows, first row is with c0 at 300Mhz, second with c0 at 1.9GHz. Both rows have cpu4 at 2.3GHz max time is in microseconds) ------------------------------------------------------| c0 max time| c0 lock count| c4 max time| c4 lock count| ------------------------------------------------------| 999843| 25| 2| 12988498| -> c0/cpu0 at 300Mhz 0| 8421132| 1| 9152979| -> c0/cpu0 at 1.9GHz ------------------------------------------------------| 999860| 160| 2| 12963487| 1| 8418492| 1| 9158001| ------------------------------------------------------| 999381| 734| 2| 12988636| 1| 8387562| 1| 9128056| ------------------------------------------------------| 989800| 750| 3| 12996473| 1| 8389091| 1| 9112444| ------------------------------------------------------|
cpu_relax() patch with __delay(1): (pairs of rows, first row is with c0 at 300Mhz, second with c0 at 1.9GHz. Both rows have cpu4 at 2.3GHz. max time is in microseconds) ------------------------------------------------------| c0 max time| c0 lock count| c4 max time| c4 lock count| ------------------------------------------------------| 7703| 1532| 2| 13035203| -> c0/cpu0 at 300Mhz 1| 8511686| 1| 8550411| -> c0/cpu0 at 1.9GHz ------------------------------------------------------| 7801| 1561| 2| 13040188| 1| 8553985| 1| 8609853| ------------------------------------------------------| 3953| 1576| 2| 13049991| 1| 8576370| 1| 8611533| ------------------------------------------------------| 3953| 1557| 2| 13030553| 1| 8509020| 1| 8543883| ------------------------------------------------------|
I should also note that my earlier kernel was 4.9-stable based and the one above was on a 4.4-stable based kernel.
Thanks, Vikram
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