Re: [PATCH 3/4] mm/memcg: Add a local_lock_t for IRQ and TASK object.

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2022-01-26 16:20:36 [+0100], Michal Hocko wrote:
> I do not see any obvious problem with this patch. The code is ugly as
> hell, though, but a large part of that is because of the weird locking
> scheme we already have. I've had a look at 559271146efc ("mm/memcg:
> optimize user context object stock access") and while I agree that it
> makes sense to optimize for user context I do not really see any numbers
> justifying the awkward locking scheme. Is this complexity really worth
> it?

From https://https://lkml.kernel.org/r/.kernel.org/all/YdX+INO9gQje6d0S@linutronix.de/:

|        Sandy Bridge   Haswell        Skylake         AMD-A8 7100    Zen2           ARM64
|PREEMPT 5,123,896,822  5,215,055,226   5,077,611,590  6,012,287,874  6,234,674,489  20,000,000,100
|IRQ     7,494,119,638  6,810,367,629  10,620,130,377  4,178,546,086  4,898,076,012  13,538,461,925

basically if PREEMPT < IRQ then preempt_disable() + enable() was cheaper
than local_irq_save() + restore().

| Sandy Bridge
|                  SERVER OPT   SERVER NO-OPT    PREEMPT OPT     PREEMPT NO-OPT
| ALLOC/FREE    8,519,295,176   9,051,200,652    10,627,431,395  11,198,189,843
| SD                5,309,768      29,253,976       129,102,317      40,681,909
| ALLOC/FREE BH 9,996,704,330   8,927,026,031    11,680,149,900  11,139,356,465
| SD               38,237,534      72,913,120        23,626,932     116,413,331

OPT is code as-is while "NO-OPT" is with the following patch which
disables the optimisation (so it should be a revert of the optimisation
commit).

ALLOC/FREE is kfree(kmalloc()).
ALLOC/FREE BH is the same but in_interrupt() reported true.
The numbers are are time needed in ns for 100,000,000 iterations of the
free+alloc. SD is standard deviation.
I also let the test run on a Zen2 box:

|                  SERVER OPT   SERVER NO-OPT   PREEMPT OPT      PREEMPT NO-OPT
| ALLOC/FREE    8,126,735,313   8,751,307,383    9,822,927,142   10,045,105,425
| SD              100,806,471      87,234,047       55,170,179       25,832,386
| ALLOC/FREE BH 9,197,455,885   8,394,337,053   10,671,227,095    9,904,954,934
| SD              155,223,919      57,800,997       47,529,496      105,260,566

Is this what you asked for?

Sebastian