linux-kernel - Re: [PATCH] alloc_tag: introduce Kconfig option for default compressed profiling

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Message-ID: <ogiahgnqihzg6nfew6oifgzjvetiwo25utp6jwskrhw4axdevp@rwqbgeraivym>
Date: Thu, 17 Apr 2025 08:47:29 -0700
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: Usama Arif <usamaarif642@...il.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org, hannes@...xchg.org, 
	linux-kernel@...r.kernel.org, kernel-team@...a.com
Subject: Re: [PATCH] alloc_tag: introduce Kconfig option for default
 compressed profiling

On Wed, Apr 16, 2025 at 05:11:11PM -0700, Suren Baghdasaryan wrote:
> On Wed, Apr 16, 2025 at 2:41 PM Shakeel Butt <shakeel.butt@...ux.dev> wrote:
> >
> > On Wed, Apr 16, 2025 at 02:08:31PM -0700, Suren Baghdasaryan wrote:
> > > On Wed, Apr 16, 2025 at 11:06 AM Usama Arif <usamaarif642@...il.com> wrote:
> > > >
> > > > With this Kconfig option enabled, the kernel stores allocation tag references
> > > > in the page flags by default.
> > > >
> > > > There are 2 reasons to introduce this:
> > > > - As mentioned in [1], compressed tags dont have system memory overhead
> > > > and much lower performance overhead. It would be preferrable to have this as
> > > > the default option, and to be able to switch it at compile time. Another
> > > > option is to just declare the static key as true by default?
> > > > - As compressed option is the best one, it doesn't make sense to have to
> > > > change both defconfig and command line options to enable memory
> > > > allocation profiling. Changing commandline across a large number of services
> > > > can result in signifcant work, which shouldn't be needed if the kernel
> > > > defconfig needs to be changed anyways.
> > >
> > > The reason tag compression is not the default option is because it
> > > works only if there are enough free bits in the page flags to store a
> > > tag index. If you configure it to use page flags and your build does
> > > not have enough free bits, the profiling will be disabled (see
> > > alloc_tag_sec_init()).
> >
> > Is it possible to fail the build in that case i.e. check the page flags
> > availability at build time?
> 
> The difficulty is finding out the number of allocation tags in the
> kernel before it gets built. Maybe there is a way to add an additional
> post-build stage to run that check.

Yeah that would be good to have.

> But even then making this option
> default and causing build failures does not seem like a good idea to
> me but maybe I'm being too cautious?

Oh my question was orthogonal to the patch. Basically some users may
want build time guarantee for this and they can enable such
build-failing opt-in config/check.