linux-kernel - Re: [PATCH] mm: append __GFP_COMP flag for trace

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Date:   Tue, 27 Apr 2021 04:36:32 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Xiongwei Song <sxwjean@...il.com>
Cc:     Xiongwei Song <sxwjean@...com>, cl@...ux.com, penberg@...nel.org,
        rientjes@...gle.com, iamjoonsoo.kim@....com,
        akpm@...ux-foundation.org, vbabka@...e.cz, linux-mm@...ck.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: append __GFP_COMP flag for trace_malloc

On Tue, Apr 27, 2021 at 11:29:32AM +0800, Xiongwei Song wrote:
> On Tue, Apr 27, 2021 at 10:54 AM Matthew Wilcox <willy@...radead.org> wrote:
> >
> > On Tue, Apr 27, 2021 at 10:43:20AM +0800, Xiongwei Song wrote:
> > > From: Xiongwei Song <sxwjean@...il.com>
> > >
> > > When calling kmalloc_order, the flags should include __GFP_COMP here,
> > > so that trace_malloc can trace the precise flags.
> >
> > I suppose that depends on your point of view.
> Correct.
> 
> Should we report the
> > flags used by the caller, or the flags that we used to allocate memory?
> > And why does it matter?
> When I capture kmem:kmalloc events on my env with perf:
> (perf record -p my_pid -e kmem:kmalloc)
> I got the result below:
>      0.08%  call_site=ffffffff851d0cb0 ptr=0xffff8c04a4ca0000
> bytes_req=10176 bytes_alloc=16384
> gfp_flags=GFP_ATOMIC|__GFP_NOWARN|__GFP_NOMEMALLOC

Hmm ... if you have a lot of allocations about this size, that would
argue in favour of adding a kmem_cache of 10880 [*] bytes.  That way,
we'd get 3 allocations per 32kB instead of 2.

[*] 32768 / 3, rounded down to a 64 byte cacheline

But I don't understand why this confused you.  Your caller at
ffffffff851d0cb0 didn't specify __GFP_COMP.  I'd be more confused if
this did report __GFP_COMP.