linux-kernel - Re: [PATCH] lockref: Limit number of cmpxchg loop retries

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Date:   Fri, 7 Jun 2019 13:14:08 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Jan Glauber <jglauber@...vell.com>
Cc:     Will Deacon <will.deacon@....com>,
        Jan Glauber <jglauber@...ium.com>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Jayachandran Chandrasekharan Nair <jnair@...vell.com>
Subject: Re: [PATCH] lockref: Limit number of cmpxchg loop retries

On Fri, Jun 7, 2019 at 12:27 AM Jan Glauber <jglauber@...vell.com> wrote:
>
> To clarify, with 224 threads & CPUs queued_spin_lock_slowpath is the top hit
> even without a retry limit in lockref. This could be unrelated to the lockref
> fallback, it looks like it's coming from the spinlock in:
>         do_sys_open -> get_unused_fd_flags -> __alloc_fd

At some point I stop worrying about microbenchmarks just because it's
easy to hit some locking paths in them, without it necessarily being
relevant in real loads.

But I'll apply the lockref patch because I think the "limit cmpxchg
loop" is conceptually a valid model, and while I think the "hitting
the same dentry lockref over and over again" is likely also an
artifact of a microbenchmark, I could at least imagine that it happens
with some common dentries (root, cwd) in some situations.

                   Linus