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

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Message-ID: <CAJuCfpHcUfXb8RaGCExLc5U2HK3xcA0CjY1diM=S6DDqNFFykg@mail.gmail.com>
Date: Wed, 16 Apr 2025 17:11:11 -0700
From: Suren Baghdasaryan <surenb@...gle.com>
To: Shakeel Butt <shakeel.butt@...ux.dev>
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 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. 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?