lkml.org 
[lkml]   [2021]   [Jan]   [9]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
SubjectRE: [PATCH] iov_iter: optimise iter type checking
Date
From: Al Viro
> Sent: 09 January 2021 17:04
>
> On Sat, Jan 09, 2021 at 04:09:08PM +0000, Pavel Begunkov wrote:
> > On 06/12/2020 16:01, Pavel Begunkov wrote:
> > > On 21/11/2020 14:37, 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
> > >> compilers, so even if some cases were handled separately with
> > >> iov_iter_is_*(), corresponding ifs in iterate*() right after are not
> > >> eliminated.
> > >>
> > >> E.g. iov_iter_npages() first handles discards, but iterate_all_kinds()
> > >> still checks for discard iter type and generates unreachable code down
> > >> the line.
> > >
> > > Ping. This one should be pretty simple
> >
> > Ping please. Any doubts about this patch?
>
> Sorry, had been buried in other crap. I'm really not fond of the
> bitmap use; if anything, I would rather turn iterate_and_advance() et.al.
> into switches...

That loses any optimisations in the order of the comparisons.
The bitmap also allows different groups to be optimised for in different code paths.

> How about moving the READ/WRITE part into MSB? Checking is just as fast
> (if not faster - check for sign vs. checking bit 0). And turn the
> types into straight (dense) enum.

Does any code actually look at the fields as a pair?
Would it even be better to use separate bytes?
Even growing the on-stack structure by a word won't really matter.

David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)

\
 
 \ /
  Last update: 2021-01-09 22:53    [W:0.095 / U:0.040 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site