linux-kernel - Re: [PATCH 00/40] Memory allocation profiling

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Date:   Wed, 3 May 2023 20:33:28 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     Kent Overstreet <kent.overstreet@...ux.dev>,
        Johannes Weiner <hannes@...xchg.org>,
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Subject: Re: [PATCH 00/40] Memory allocation profiling

On Wed, May 3, 2023 at 7:25 PM Tejun Heo <tj@...nel.org> wrote:
>
> Hello,
>
> On Wed, May 03, 2023 at 01:14:57PM -0700, Suren Baghdasaryan wrote:
> > On Wed, May 3, 2023 at 1:00 PM Tejun Heo <tj@...nel.org> wrote:
> > > Another related question. So, the reason for macro'ing stuff is needed is
> > > because you want to print the line directly from kernel, right?
> >
> > The main reason is because we want to inject a code tag at the
> > location of the call. If we have a code tag injected at every
> > allocation call, then finding the allocation counter (code tag) to
> > operate takes no time.
> >
> > > Is that
> > > really necessary? Values from __builtin_return_address() can easily be
> > > printed out as function+offset from kernel which already gives most of the
> > > necessary information for triaging and mapping that back to source line from
> > > userspace isn't difficult. Wouldn't using __builtin_return_address() make
> > > the whole thing a lot simpler?
> >
> > If we do that we have to associate that address with the allocation
> > counter at runtime on the first allocation and look it up on all
> > following allocations. That introduces the overhead which we are
> > trying to avoid by using macros.
>
> I see. I'm a bit skeptical about the performance angle given that the hot
> path can be probably made really cheap even with lookups. In most cases,
> it's just gonna be an extra pointer deref and a few more arithmetics. That
> can show up in microbenchmarks but it's not gonna be much. The benefit of
> going that route would be the tracking thing being mostly self contained.

I'm in the process of rerunning the tests to compare the overhead on
the latest kernel but I don't expect that to be cheap compared to
kmalloc().

>
> That said, it's nice to not have to worry about allocating tracking slots
> and managing hash table, so no strong opinion.
>
> Thanks.
>
> --
> tejun
>
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