linux-kernel - RE: [PATCH v2 3/3] rseq: optimise rseq_get_rseq_cs() and clear_rseq

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Message-ID: <feed2c13dbe34279a03929a588c46c67@AcuMS.aculab.com>
Date:   Tue, 13 Apr 2021 21:19:10 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Arjun Roy' <arjunroy@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>
CC:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        paulmck <paulmck@...nel.org>, Boqun Feng <boqun.feng@...il.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v2 3/3] rseq: optimise rseq_get_rseq_cs() and
 clear_rseq_cs()

> If we're special-casing 64-bit architectures anyways - unrolling the
> 32B copy_from_user() for struct rseq_cs appears to be roughly 5-10%
> savings on x86-64 when I measured it (well, in a microbenchmark, not
> in rseq_get_rseq_cs() directly). Perhaps that could be an additional
> avenue for improvement here.

The killer is usually 'user copy hardening'.
It significantly slows down sendmsg() and recvmsg().
I've got measurable performance improvements by
using __copy_from_user() when the buffer since has
already been checked - but isn't a compile-time constant.

There is also scope for using _get_user() when reading
iovec[] (instead of copy_from_user()) and doing all the
bound checks (etc) in the loop.
That gives a measurable improvement for writev("/dev/null").
I must sort those patches out again.

	David

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