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

[Xiongwei Song]

Hi Mattew,

One more thing I should explain, the kmalloc_order() appends the
__GFP_COMP flags,
not by the caller.

void *kmalloc_order(size_t size, gfp_t flags, unsigned int order)
{
...........................................................

flags |= __GFP_COMP;
page = alloc_pages(flags, order);
...........................................................
return ret;
}
EXPORT_SYMBOL(kmalloc_order);

#ifdef CONFIG_TRACING
void *kmalloc_order_trace(size_t size, gfp_t flags, unsigned int order)
{
void *ret = kmalloc_order(size, flags, order);
trace_kmalloc(_RET_IP_, ret, size, PAGE_SIZE << order, flags);
return ret;
}
EXPORT_SYMBOL(kmalloc_order_trace);
#endif


Regards,
Xiongwei

On Tue, Apr 27, 2021 at 12:11 PM Xiongwei Song <sxwjean@xxxxxxxxx> wrote:
>
> On Tue, Apr 27, 2021 at 11:36 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> >
> > 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.
> I understand you. But I don't think our process needs this size. This size
> may be a bug in our code or somewhere, I don't know the RC for now.
>
> > [*] 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.
> >
> I just wanted to save some time when debugging.
>
> Regards