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From: Al Viro <viro@ZenIV.linux.org.uk>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Eric Dumazet <eric.dumazet@gmail.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: Linux 4.9-rc6
Date: Mon, 21 Nov 2016 01:35:58 +0000	[thread overview]
Message-ID: <20161121013558.GG1555@ZenIV.linux.org.uk> (raw)
In-Reply-To: <CA+55aFzpTrPgszdD32rJ=WgGjjLJ_bjvgegQuW=82phNE8kvLQ@mail.gmail.com>

On Sun, Nov 20, 2016 at 03:27:07PM -0800, Linus Torvalds wrote:
> On Sun, Nov 20, 2016 at 2:27 PM, Eric Dumazet <eric.dumazet@gmail.com> wrote:
> >
> > Hosts with ~100,000 threads have an issue with /prov/vmallocinfo
> >
> > It can take about 800 usec to skip over ~100,000 struct vmap_area
> > in s_start(), while holding vmap_area_lock spinlock, and therefore
> > blocking fork()/pthread_create().
> >
> > I presume we can not switch to the rbtree (vmap_area_root)
> > for /proc/vmallocinfo, because this file is seek-able, right ?
> 
> Well, the good news is that the file is root-only anyway, which means
> that at least it won't have the issue that a lot of other /proc files
> have had - namely being opened by random user programs or libraries.
> 
> Which means that the users of it are likely fairly limited.
> 
> Which in turn means that we can probably afford to play more games
> with it. Including, for example, possibly marking it non-seekable.
> 
> Or even just limit the maximum entries we are willing to walk.
> 
> Or we could decide that that file shouldn't be a seq_file at all, use
> the old "one page buffer" approach that was so common for /proc files,
> and make the position encode the vmalloc address in it (make the lower
> PAGE_MASK bits be the offset in the line), and then we *could* just
> look things up using the btree method.
> 
> Al, do you have any clever ideas?

Umm...  One possibility would be something like fs/namespace.c:m_start() -
if nothing has changed since the last time, just use a cached pointer.
That has sped the damn thing (/proc/mounts et.al.) big way, but it's
dependent upon having an event count updated whenever we change the
mount tree - doing the same for vma_area list might or might not be
a good idea.  /proc/mounts and friends get ->poll() on that as well;
that probably would _not_ be a good idea in this case.

  reply	other threads:[~2016-11-21  1:36 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-11-20 22:05 Linus Torvalds
2016-11-20 22:27 ` Eric Dumazet
2016-11-20 23:27   ` Linus Torvalds
2016-11-21  1:35     ` Al Viro [this message]
2016-11-21  4:59       ` Eric Dumazet
2016-11-21  8:34         ` David Rientjes
2016-11-21 13:32           ` Eric Dumazet
2016-11-21 13:51             ` Eric Dumazet
2016-11-21 16:49               ` Eric Dumazet
2016-12-04 10:43               ` Thorsten Leemhuis
     [not found]                 ` <CA+55aFzPiZW4FfWbvM-+AFraa0fkUHv4C1Y9SCzHdXEcUSPqdg@mail.gmail.com>
2016-12-04 17:17                   ` Eric Dumazet
2016-12-21 15:30                     ` Eric Dumazet

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