linux-kernel - Re: [PATCH v2 1/3] fs/file.c: add fast path in alloc

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Message-ID: <3d553b6571eaa878e4ce68898113d73c9c1ed87d.camel@linux.intel.com>
Date: Wed, 26 Jun 2024 09:52:50 -0700
From: Tim Chen <tim.c.chen@...ux.intel.com>
To: Jan Kara <jack@...e.cz>, "Ma, Yu" <yu.ma@...el.com>
Cc: viro@...iv.linux.org.uk, brauner@...nel.org, mjguzik@...il.com, 
 edumazet@...gle.com, linux-fsdevel@...r.kernel.org,
 linux-kernel@...r.kernel.org,  pan.deng@...el.com, tianyou.li@...el.com,
 tim.c.chen@...el.com
Subject: Re: [PATCH v2 1/3] fs/file.c: add fast path in alloc_fd()

On Wed, 2024-06-26 at 09:43 -0700, Tim Chen wrote:
> On Wed, 2024-06-26 at 13:54 +0200, Jan Kara wrote:
> > 
> > 
> > Indeed, thanks for correcting me! next_fd is just a lower bound for the
> > first free fd.
> > 
> > > The conditions
> > > should either be like it is in patch or if (!start && !test_bit(0,
> > > fdt->full_fds_bits)), the latter should also have the bitmap loading cost,
> > > but another point is that a bit in full_fds_bits represents 64 bits in
> > > open_fds, no matter fd >64 or not, full_fds_bits should be loaded any way,
> > > maybe we can modify the condition to use full_fds_bits ?
> > 
> > So maybe I'm wrong but I think the biggest benefit of your code compared to
> > plain find_next_fd() is exactly in that we don't have to load full_fds_bits
> > into cache. So I'm afraid that using full_fds_bits in the condition would
> > destroy your performance gains. Thinking about this with a fresh head how
> > about putting implementing your optimization like:
> > 
> > --- a/fs/file.c
> > +++ b/fs/file.c
> > @@ -490,6 +490,20 @@ static unsigned int find_next_fd(struct fdtable *fdt, unsigned int start)
> >         unsigned int maxbit = maxfd / BITS_PER_LONG;
> >         unsigned int bitbit = start / BITS_PER_LONG;
> >  
> > +       /*
> > +        * Optimistically search the first long of the open_fds bitmap. It
> > +        * saves us from loading full_fds_bits into cache in the common case
> > +        * and because BITS_PER_LONG > start >= files->next_fd, we have quite
> > +        * a good chance there's a bit free in there.
> > +        */
> > +       if (start < BITS_PER_LONG) {
> > +               unsigned int bit;
> > +
> > +               bit = find_next_zero_bit(fdt->open_fds, BITS_PER_LONG, start);
> 
> Say start is 31 (< BITS_PER_LONG)
> bit found here could be 32 and greater than start.  Do we care if we return bit > start?

Sorry, I mean to say that we could find a bit like 30 that is less than start instead
of the other way round. 

Tim
>