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SubjectRe: [RFC v2 00/34] SLUB: reduce irq disabled scope and make it RT compatible
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On 7/2/21 8:29 PM, Sebastian Andrzej Siewior wrote:
> I replaced my slub changes with slub-local-lock-v2r3.
> I haven't seen any complains from lockdep or so which is good. Then I
> did this with RT enabled (and no debug):

Thanks for testing!

> - A "time make -j32" run of allmodconfig on /dev/shm.
> Old:
> | real 20m6,217s
> | user 568m22,553s
> | sys 48m33,126s
>
> New:
> | real 20m9,049s
> | user 569m32,096s
> | sys 48m47,670s
>
> These 3 seconds here are probably in the noise range.
>
> - perf_5.10 stat -r 10 hackbench -g200 -s 4096 -l500
> Old:
> | 464.967,20 msec task-clock # 27,220 CPUs utilized ( +- 0,16% )
> | 7.683.944 context-switches # 0,017 M/sec ( +- 0,86% )
> | 931.380 cpu-migrations # 0,002 M/sec ( +- 4,94% )
> | 219.569 page-faults # 0,472 K/sec ( +- 0,39% )
> | 1.104.727.599.918 cycles # 2,376 GHz ( +- 0,18% )
> | 941.428.898.087 stalled-cycles-frontend # 85,22% frontend cycles idle ( +- 0,24% )
> | 729.016.546.572 stalled-cycles-backend # 65,99% backend cycles idle ( +- 0,32% )
> | 340.133.571.519 instructions # 0,31 insn per cycle
> | # 2,77 stalled cycles per insn ( +- 0,12% )
> | 73.746.821.314 branches # 158,607 M/sec ( +- 0,13% )
> | 377.838.006 branch-misses # 0,51% of all branches ( +- 1,01% )
> |
> | 17,0820 +- 0,0202 seconds time elapsed ( +- 0,12% )
>
> New:
> | 422.865,71 msec task-clock # 4,782 CPUs utilized ( +- 0,34% )
> | 14.594.238 context-switches # 0,035 M/sec ( +- 0,43% )
> | 3.737.926 cpu-migrations # 0,009 M/sec ( +- 0,46% )
> | 218.474 page-faults # 0,517 K/sec ( +- 0,74% )
> | 940.715.812.020 cycles # 2,225 GHz ( +- 0,34% )
> | 716.593.827.820 stalled-cycles-frontend # 76,18% frontend cycles idle ( +- 0,39% )
> | 550.730.862.839 stalled-cycles-backend # 58,54% backend cycles idle ( +- 0,43% )
> | 417.274.588.907 instructions # 0,44 insn per cycle
> | # 1,72 stalled cycles per insn ( +- 0,17% )
> | 92.814.150.290 branches # 219,488 M/sec ( +- 0,17% )
> | 822.102.170 branch-misses # 0,89% of all branches ( +- 0,41% )
> |
> | 88,427 +- 0,618 seconds time elapsed ( +- 0,70% )
>
> So this is outside of the noise range.
> I'm not sure where this is coming from. My guess would be higher lock
> contention within the memory allocator.

The series shouldn't significantly change the memory allocator
interaction, though.
Seems there's less cycles, but more time elapsed, thus more sleeping -
is it locks becoming mutexes on RT?

My first guess - the last, local_lock patch. What would happen if you
take that one out? Should be still RT-compatible. If it improves a lot,
maybe that conversion to local_lock is not worth it then.

My second guess - list_lock remains spinlock with my series, thus RT
mutex, but the current RT tree converts it to raw_spinlock. I'd hope
leaving that one as non-raw spinlock would still be much better for RT
goals, even if hackbench (which is AFAIK very slab intensive) throughput
regresses - hopefully not that much.

>> The remaining patches to upstream from the RT tree are small ones related to
>> KConfig. The patch that restricts PREEMPT_RT to SLUB (not SLAB or SLOB) makes
>> sense. The patch that disables CONFIG_SLUB_CPU_PARTIAL with PREEMPT_RT could
>> perhaps be re-evaluated as the series also addresses some latency issues with
>> percpu partial slabs.
>
> With that series the PARTIAL slab can be indeed enabled. I have (had) a
> half done series where I had PARTIAL enabled and noticed a slight
> increase in latency so made it "default y on !RT". It wasn't dramatic
> but appeared to be outside of noise.
>
> Sebastian
>

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