Re: [PATCH] mm, oom: allow oom reaper to race with exit_mmap

[Michal Hocko <mhocko@kernel.org>]

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On Mon 24-07-17 18:11:46, Michal Hocko wrote:
> On Mon 24-07-17 17:51:42, Kirill A. Shutemov wrote:
> > On Mon, Jul 24, 2017 at 04:15:26PM +0200, Michal Hocko wrote:
> [...]
> > > What kind of scalability implication you have in mind? There is
> > > basically a zero contention on the mmap_sem that late in the exit path
> > > so this should be pretty much a fast path of the down_write. I agree it
> > > is not 0 cost but the cost of the address space freeing should basically
> > > make it a noise.
> > 
> > Even in fast path case, it adds two atomic operation per-process. If the
> > cache line is not exclusive to the core by the time of exit(2) it can be
> > noticible.
> > 
> > ... but I guess it's not very hot scenario.
> > 
> > I guess I'm just too cautious here. :)
> 
> I definitely did not want to handwave your concern. I just think we can
> rule out the slow path and didn't think about the fast path overhead.
> 
> > > > Should we do performance/scalability evaluation of the patch before
> > > > getting it applied?
> > > 
> > > What kind of test(s) would you be interested in?
> > 
> > Can we at lest check that number of /bin/true we can spawn per second
> > wouldn't be harmed by the patch? ;)
> 
> OK, so measuring a single /bin/true doesn't tell anything so I've done
> root@test1:~# cat a.sh 
> #!/bin/sh
> 
> NR=$1
> for i in $(seq $NR)
> do
>         /bin/true
> done

I wanted to reduce a potential shell side effects so I've come with a
simple program which forks and saves the timestamp before child exit and
right after waitpid (see attached) and then measured it 100k times. Sure
this still measures waitpid overhead and the signal delivery but this
should be more or less constant on an idle system, right? See attached.

before the patch
min: 306300.00 max: 6731916.00 avg: 437962.07 std: 92898.30 nr: 100000

after
min: 303196.00 max: 5728080.00 avg: 436081.87 std: 96165.98 nr: 100000

The results are well withing noise as I would expect.
-- 
Michal Hocko
SUSE Labs

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#include <sys/mman.h>
#include <sys/wait.h>
#include <sys/time.h>
#include <assert.h>
#include <stdlib.h>
#include <stdio.h>
#include <inttypes.h>
#include <unistd.h>

#define NR_FORKS 100000

static inline uint64_t get_cycles(void)
{
	uint64_t t;
	volatile int dont_remove __attribute__((unused));
	unsigned tmp;

	__asm volatile ("cpuid" : "=a"(tmp), "=b"(tmp), "=c"(tmp), "=d"(tmp): "a" (0));

	dont_remove = tmp; 
	__asm volatile ("rdtsc" : "=A"(t));
	return t;
}

int main(int argc, char **argv)
{
	void *addr = mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_ANON|MAP_SHARED, -1, 0);
	int i = NR_FORKS, j = 1;

	assert(addr != MAP_FAILED);

	while (i-- > 0) {
		pid_t child = fork();
		uint64_t before, after;

		assert(child != -1);
		if (!child) {
			*(uint64_t *)addr = get_cycles();
			return 0;
		}
		assert(child == waitpid(child, NULL, 0));
		before = *(uint64_t *)addr;
		after = get_cycles();

		printf("%u\n", (unsigned)(after - before));
		fflush(stdout);
	}

	return 0;
}