Date:	Thu, 2 Jul 2009 18:30:04 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Paul Menage <menage@...gle.com>
Cc:	Benjamin Blum <bblum@...gle.com>, lizf@...fujitzu.com,
	serue@...ibm.com, containers@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] Adds a read-only "procs" file similar to "tasks"
 that  shows only unique tgids

On Thu, 2 Jul 2009 18:08:29 -0700 Paul Menage <menage@...gle.com> wrote:

> On Thu, Jul 2, 2009 at 5:53 PM, Andrew Morton<akpm@...ux-foundation.org> wrote:
> >> In the first snippet, count will be at most equal to length. As length
> >> is determined from cgroup_task_count, it can be no greater than the
> >> total number of pids on the system.
> >
> > Well that's a problem, because there can be tens or hundreds of
> > thousands of pids, and there's a fairly low maximum size for kmalloc()s
> > (include/linux/kmalloc_sizes.h).
> >
> > And even if this allocation attempt doesn't exceed KMALLOC_MAX_SIZE,
> > large allocations are less unreliable. __There is a large break point at
> > 8*PAGE_SIZE (PAGE_ALLOC_COSTLY_ORDER).
> 
> This has been a long-standing problem with the tasks file, ever since
> the cpusets days.
> 
> There are ways around it - Lai Jiangshan <laijs@...fujitsu.com> posted
> a patch that allocated an array of pages to store pids in, with a
> custom sorting function that let you specify indirection rather than
> assuming everything was in one contiguous array. This was technically
> the right approach in terms of not needing vmalloc and never doing
> large allocations, but it was very complex; an alternative that was
> mooted was to use kmalloc for small cgroups and vmalloc for large
> ones, so the vmalloc penalty wouldn't be paid generally. The thread
> fizzled AFAICS.

It's a problem which occurs fairly regularly.  Some sites are fairly
busted.  Many gave up and used vmalloc().  Others use an open-coded
array-of-pages thing.

This happens enough that I expect the kernel would benefit from a
general dynamic-array library facility.  Something whose interface
mimics the C-level array operations but which is internally implemented
via some data structure which uses PAGE_SIZE allocations.  Probably a
simple two-level thing would suffice.

> >
> > One could perhaps create an alias (symlink?) and leave that in place
> > for a few kernel releases and then remove the old names. __The trick to
> > doing this politely is to arrange for a friendly printk to come out
> > when userspace uses the old filename, so people know to change their
> > tools. __That printk should come out once-per-boot, not once-per-access.
> 
> Personally, I feel that a bit of ugliness in the naming inconsistency
> is less painful than trying to deprecate something that people might
> be using. If we could just flip the names without breaking anyone,
> that would be great, but this is just a style issue rather than a
> functional issue.

Sure, leaving things as they are won't kill us.

But I do expect it'd be pretty easy to migrate to new names.

> My experience of such printk() statements scattered
> around in code is that no-one takes much notice of them.

mm...  I don't recall having any problems with the approach, the few
times we've tried it.  This case is particularly easy because at this
stage the audience is developers, and usually developers who wrote
their own stuff and don't have to wait for providers/distros/etc to
make changes.

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