Re: [PATCH] mm: append __GFP_COMP flag for trace_malloc

[Matthew Wilcox]

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@xxxxxxxxxxxxx> wrote:
> >
> > On Tue, Apr 27, 2021 at 10:43:20AM +0800, Xiongwei Song wrote:
> > > From: Xiongwei Song <sxwjean@xxxxxxxxx>
> > >
> > > 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.