From: Al Viro <viro@zeniv.linux.org.uk>
To: Pavel Begunkov <asml.silence@gmail.com>
Cc: Christoph Hellwig <hch@infradead.org>,
Jens Axboe <axboe@kernel.dk>,
linux-block@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/2] iov_iter: optimise iter type checking
Date: Fri, 11 Dec 2020 02:01:00 +0000 [thread overview]
Message-ID: <20201211020100.GB107834@ZenIV.linux.org.uk> (raw)
In-Reply-To: <ce79f47e-2ec0-ba29-a991-c537a8990dee@gmail.com>
On Thu, Nov 19, 2020 at 05:12:44PM +0000, Pavel Begunkov wrote:
> On 19/11/2020 17:03, Christoph Hellwig wrote:
> > On Thu, Nov 19, 2020 at 03:29:43PM +0000, Pavel Begunkov wrote:
> >> The problem here is that iov_iter_is_*() helpers check types for
> >> equality, but all iterate_* helpers do bitwise ands. This confuses
> >> a compiler, so even if some cases were handled separately with
> >> iov_iter_is_*(), it can't eliminate and skip unreachable branches in
> >> following iterate*().
> >
> > I think we need to kill the iov_iter_is_* helpers, renumber to not do
> > the pointless bitmask and just check for equality (might turn into a
> > bunch of nice switch statements actually).
>
> There are uses like below though, and that would also add some overhead
> on iov_iter_type(), so it's not apparent to me which version would be
> cleaner/faster in the end. But yeah, we can experiment after landing
> this patch.
>
> if (type & (ITER_BVEC|ITER_KVEC))
There are exactly 3 such places, and all of them would've been just as well
with case ITER_BVEC: case ITER_KVEC: ... in a switch.
Hmm... I wonder which would work better:
enum iter_type {
ITER_IOVEC = 0,
ITER_KVEC = 2,
ITER_BVEC = 4,
ITER_PIPE = 6,
ITER_DISCARD = 8,
};
iov_iter_type(iter) (((iter)->type) & ~1)
iov_iter_rw(iter) (((iter)->type) & 1)
or
enum iter_type {
ITER_IOVEC,
ITER_KVEC,
ITER_BVEC,
ITER_PIPE,
ITER_DISCARD,
};
iov_iter_type(iter) (((iter)->type) & (~0U>>1))
// callers of iov_iter_rw() are almost all comparing with explicit READ or WRITE
iov_iter_rw(iter) (((iter)->type) & ~(~0U>>1) ? WRITE : READ)
with places like iov_iter_kvec() doing
i->type = ITER_KVEC | ((direction == WRITE) ? BIT(31) : 0);
Preferences?
next prev parent reply other threads:[~2020-12-11 2:02 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-19 15:29 [PATCH 0/2] optimise iov_iter Pavel Begunkov
2020-11-19 15:29 ` [PATCH 1/2] iov_iter: optimise iov_iter_npages for bvec Pavel Begunkov
2020-11-19 15:29 ` [PATCH 2/2] iov_iter: optimise iter type checking Pavel Begunkov
2020-11-19 17:03 ` Christoph Hellwig
2020-11-19 17:12 ` Pavel Begunkov
2020-12-11 2:01 ` Al Viro [this message]
2020-12-13 22:32 ` Pavel Begunkov
2020-12-14 10:28 ` David Laight
2020-12-14 17:25 ` Pavel Begunkov
2020-11-19 16:46 ` [PATCH 0/2] optimise iov_iter Jens Axboe
2020-11-19 17:14 ` Pavel Begunkov
2020-11-19 17:20 ` Jens Axboe
2020-11-19 18:02 ` Pavel Begunkov
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