[LTP] [PATCH v2 1/3] mem: child alloc memory should larger than memory.max + memory.swap.max if lite==1

[Richard Palethorpe]

Hello Li,

Li Wang <liwang@redhat.com> writes:

> oom03 often gets fail while setting 'memory.swap.max = TESTMEM' in CGroup V2,
> as in that scenario (lite == 1), child_alloc only start a single process to
> dirty 'TESTMEM + MB' anonymous memory for testing:
>
> testoom(, lite == 1, ,)
>   oom(, lite == 1, ,)
>     child_alloc(, lite == 1,)
>         alloc_mem(TESTMEM + MB, )
>
>   mem.c:224: TINFO: start normal OOM testing.
>   mem.c:146: TINFO: expected victim is 80466.
>   mem.c:38: TINFO: thread (7f411c69d740), allocating 1074790400 bytes.
>   mem.c:64: TINFO: swapped is 25546752 bytes.     <------- swap occuring -----
>   mem.c:164: TFAIL: victim unexpectedly ended with retcode: 0, expected: 12
>
> TBH, this can not really test the 'memory.swap.max' as expected, since in the
> kernel side mem_cgroup_out_of_memory split OOM margin into two-part, one for
> memory.max limit, another for memory.swap.max, if any of them get overflow,
> then invoke out_of_memory to kill victim-process.
>
> Theoretically, alloc_mem(TESTMEM + MB, ) should work while 'memory.max' is equal
> to TESTMEM, but Cgroup v2 tracks memory and swap in separate, which splits memory
> and swap counter. So with swappiness enable (default value is 60 on RHEL), it
> likely has part of memory swapping out during the allocating, upon that the two
> limit loss effect at the same time. Unless disable swap completely then memory.max
> will take effect in precisely.
>
> To get more opportunities to reach the swap limitation, let's scale down the
> value of 'memory.swap.max' to only 1MB for CGroup v2.
>
> But for CGroup v1, the memory.memsw.limit_in_bytes disallow to less than
> memory.limit_in_bytes, so we'd better raise the child_alloc to the
> twifold
  ^twofold
> of TESTMEM.

Ah, this means "memory.swap.x" and "memory.memsw.x" are not really the
same thing. This seems to be common pattern, so maybe we could translate
V2 values to V1 in the library.

If I understand correctly `memory.swap.max = memory.memsw.limit_in_bytes
- memory.limit_in_bytes`? Also "max" can be mapped to ~0UL or maybe
~0ULL when -m32 is used.

This is not important for the current patch.

>
> Signed-off-by: Li Wang <liwang@redhat.com>
> ---
>
> Notes:
>     v1 --> v2
>        * add comments for explaining why set 'memory.swap.max' to 1MB
>        * Scale down the value of 'memory.swap.max' to only 1MB for CGroup v2.
>
>  testcases/kernel/mem/lib/mem.c   |  2 +-
>  testcases/kernel/mem/oom/oom03.c | 17 ++++++++++++++++-
>  2 files changed, 17 insertions(+), 2 deletions(-)
>
> diff --git a/testcases/kernel/mem/lib/mem.c b/testcases/kernel/mem/lib/mem.c
> index 9f946b5c9..ac890491c 100644
> --- a/testcases/kernel/mem/lib/mem.c
> +++ b/testcases/kernel/mem/lib/mem.c
> @@ -78,7 +78,7 @@ static void child_alloc(int testcase, int lite, int threads)
>  	pthread_t *th;
>  
>  	if (lite) {
> -		int ret = alloc_mem(TESTMEM + MB, testcase);
> +		int ret = alloc_mem(TESTMEM * 2 + MB, testcase);
>  		exit(ret);
>  	}
>  
> diff --git a/testcases/kernel/mem/oom/oom03.c b/testcases/kernel/mem/oom/oom03.c
> index 939413744..89d7711a5 100644
> --- a/testcases/kernel/mem/oom/oom03.c
> +++ b/testcases/kernel/mem/oom/oom03.c
> @@ -46,7 +46,22 @@ static void verify_oom(void)
>  	testoom(0, 0, ENOMEM, 1);
>  
>  	if (SAFE_CGROUP_HAS(cg, "memory.swap.max")) {
> -		SAFE_CGROUP_PRINTF(cg, "memory.swap.max", "%lu", TESTMEM);
> +		/*
> +		 * Cgroup v2 tracks memory and swap in separate, which splits
> +		 * memory and swap counter. So with swappiness enable (default
> +		 * value is 60 on RHEL), it likely has part of memory swapping
> +		 * out during the allocating, upon that the two limit loss
> +		 * effect at the same time.
> +		 *
> +		 * To get more opportunities to reach the swap limitation,
> +		 * let's scale down the value of 'memory.swap.max' to only
> +		 * 1MB for CGroup v2.
> +		 */
> +		if (TST_CGROUP_VER(cg, "memory") != TST_CGROUP_V1)
> +			SAFE_CGROUP_PRINTF(cg, "memory.swap.max", "%lu", MB);
> +		else
> +			SAFE_CGROUP_PRINTF(cg, "memory.swap.max", "%lu", TESTMEM);
> +

To be consistent with V2; should this be TESTMEM + MB?

>  		testoom(0, 1, ENOMEM, 1);
>  	}
>  
> -- 
> 2.31.1


-- 
Thank you,
Richard.