Re: [PATCH v4] tracing: add 'accounted' entry into output of allocation tracepoints

Hyeonggon Yoo
2022-05-22T05:22:28Z

On Sun, May 22, 2022 at 07:33:08AM +0300, Vasily Averin wrote:
> On 5/22/22 06:51, Hyeonggon Yoo wrote:
> > On Sat, May 21, 2022 at 09:36:54PM +0300, Vasily Averin wrote:
> >> Slab caches marked with SLAB_ACCOUNT force accounting for every
> >> allocation from this cache even if __GFP_ACCOUNT flag is not passed.
> >> Unfortunately, at the moment this flag is not visible in ftrace output,
> >> and this makes it difficult to analyze the accounted allocations.
> >>
> >> This patch adds boolean "accounted" entry into trace output,
> >> and set it to 'true' for calls used __GFP_ACCOUNT flag and
> >> for allocations from caches marked with SLAB_ACCOUNT.
> >>
> >> Signed-off-by: Vasily Averin <vvs@openvz.org>
> >> Acked-by: Shakeel Butt <shakeelb@google.com>
> >
> > May I ask what information do you want to collect
> > using this patch?
>
> I analyze ftrace output to understand which allocations are accounted.
> When some userspace operation consume memory, it's important to account
> most part of memory (>2/3 of all) to avoid misuse inside memcg-limited
> contianers. Otherwise memcg-limited container can consume significant
> portion of host memory, trigger global OOM, wake up OOM-killer and kill
> random processes on host.
> If memory consumers are accounted, it leads to memcg-OOM only.
>
> Now kmem tracing output looks like this:
>
> kmem_cache_alloc: (getname_flags.part.0+0x2c) call_site=getname_flags.part.0+0x2c ptr=0xffff8fff022e9000 bytes_req=4096 bytes_alloc=4096 gfp_flags=GFP_KERNEL accounted=false
> kmalloc: (alloc_bprm+0x32) call_site=alloc_bprm+0x32 ptr=0xffff8fff2b408a00 bytes_req=416 bytes_alloc=512 gfp_flags=GFP_KERNEL|__GFP_ZERO accounted=false
> kmem_cache_alloc: (mm_alloc+0x16) call_site=mm_alloc+0x16 ptr=0xffff8fff0894d500 bytes_req=1048 bytes_alloc=1088 gfp_flags=GFP_KERNEL accounted=true
> mm_page_alloc: page=0xffffffffa4ab8d42 pfn=0x12ad72 order=1 migratetype=0 gfp_flags=GFP_USER|__GFP_ZERO|__GFP_ACCOUNT
> kmem_cache_alloc: (vm_area_alloc+0x1a) call_site=vm_area_alloc+0x1a ptr=0xffff8fff2af27000 bytes_req=200 bytes_alloc=200 gfp_flags=GFP_KERNEL accounted=true
>
> As you can see, without new field it is quite hard to understand,
> is last allocation accounted.
>
> This analyze helps me to identify most important allocations for given scenario
> and enable accounting for selected allocations.
>
> An example of this analyze you can found here:
> https://lore.kernel.org/all/d28233ee-bccb-7bc3-c2ec-461fd7f95e6a@openvz.org/
>

Thank you for detailed explanation. Makes sense to me.

> > If we decide to do that, it would be better to print
> > something like:
> > slab_flags=SLAB_RECLAIM_ACCOUNT|SLAB_ACCOUNT|SLAB_STORE_USER
> > instead of just printing 'accounted=true/false'. This patch is too
> > specific to SLAB_ACCOUNT.
>
> Any extra output degrades performance.

No strong opinion but just a concern that maybe later someone want add
something similar like 'reclaimable=true/false', 'dma=true/false', ...
and I would prefer more general solution. (especially if we'll not
change tracepoints after release because of backward compatibility)

> For my task it's not important to know SLAB flags, I just need to understand,
> is current allocation accounted or not.

SLAB_ACCOUNT, SLAB_RECLAIM_ACCOUNT, SLAB_DMA, ... etc are SLAB flags.

'if current allocation is accounted or not' depends on SLAB_ACCOUNT
flag is set or not.

Thanks,
Hyeonggon

> > And if what you want to know is just total slab memory that is accounted,
> > what about adding something like SlabAccounted in /proc/meminfo?
>
> It is not enough for me. I need to have per-process allocation information.
>
> Thank you,
> Vasily Averin