linux-kernel - Re: [RFC PATCH 10/30] mm: enable page allocation tagging for __get_free_pages and alloc

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Date:   Thu, 1 Sep 2022 09:41:02 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Kent Overstreet <kent.overstreet@...ux.dev>
Cc:     Mel Gorman <mgorman@...e.de>,
        Suren Baghdasaryan <surenb@...gle.com>,
        akpm@...ux-foundation.org, mhocko@...e.com, vbabka@...e.cz,
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Subject: Re: [RFC PATCH 10/30] mm: enable page allocation tagging for
 __get_free_pages and alloc_pages

On Wed, Aug 31, 2022 at 01:46:29PM -0400, Kent Overstreet wrote:

> Because all the counters are statically allocated, without even a pointer deref
> to get to them in the allocation path (one pointer deref to get to them in the
> deallocate path), that makes this _much, much_ cheaper than anything that could
> be done with tracing - cheap enough that I expect many users will want to enable
> it in production.

You're contributing to death-by-a-thousand-cuts here. By making all this
unconditional you're putting distros in a bind. Most of their users will
likely not care about this, but if they enable it, they'll still pay the
price for having it.

Even static counters will have cache misses etc..

So yes, for the few people that actually care about this stuff, this
might be a bit faster, but IMO it gets the econimics all backwards,
you're making everybody pay the price instead of only those that care.

Also note that you can have your tracepoint based handler have
statically allocated data just fine.