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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Sat, 12 Sep 2020 14:15:56 -0700
Message-ID: <CAHk-=wjTpK7=5Sd8i9D_FTtRSerMUP3ux4qEL+fZdDYmfLrBAg@mail.gmail.com>
Subject: Re: Kernel Benchmarking
To:     James Bottomley <James.Bottomley@hansenpartnership.com>
Cc:     Amir Goldstein <amir73il@gmail.com>,
        Hugh Dickins <hughd@google.com>,
        Michael Larabel <Michael@michaellarabel.com>,
        "Ted Ts'o" <tytso@google.com>,
        Andreas Dilger <adilger.kernel@dilger.ca>,
        Ext4 Developers List <linux-ext4@vger.kernel.org>,
        Jan Kara <jack@suse.cz>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>
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On Sat, Sep 12, 2020 at 1:59 PM James Bottomley
<James.Bottomley@hansenpartnership.com> wrote:
>
> On Sat, 2020-09-12 at 10:59 -0700, Linus Torvalds wrote:
> [...]
> > Any other suggestions than those (a)-(d) ones above?
>
> What about revert and try to fix the outliers?  Say by having a timer
> set when a process gets put to sleep waiting on the page lock.

No timer needed, I suspect.

I tried to code something like this up yesterday (hjmm. Thursday?) as
a "hybrid" scheme, where we'd start out with the old behavior and let
people unfairly get the lock while there were waiters, but when a
waiter woke up and noticed that it still couldn't get the lock, _then_
it would stat using the new scheme.

So still be unfair for a bit, but limit the unfairness so that a
waiter won't lose the lock more than once (but obviously while the
waiter initially slept, _many_ other lockers could have come through).

I ended up with a code mess and gave up on it (it seemed to just get
all the complications from the old _and_ the new model), but maybe I
should try again now that I know what went wrong last time. I think I
tried too hard to actually mix the old and the new code.

(If I tried again, I'd not try to mix the new and the old code, I'd
make the new one start out with a non-exclusive wait - which the code
already supports for that whole "wait for PG_writeback to end" as
opposed to "wait to take PG_lock" - and then turn it into an exclusive
wait if it fails.. That might work out better and not mix entirely
different approaches).

             Linus