linux-kernel - Re: [PATCH] proc: revert /proc/<pid>/maps [stack:TID] annotation

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Message-ID: <20160119233822.GA10788@cmpxchg.org>
Date:	Tue, 19 Jan 2016 18:38:22 -0500
From:	Johannes Weiner <hannes@...xchg.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Shaohua Li <shli@...com>,
	Siddhesh Poyarekar <siddhesh.poyarekar@...il.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	kernel-team@...com
Subject: Re: [PATCH] proc: revert /proc/<pid>/maps [stack:TID] annotation

On Tue, Jan 19, 2016 at 02:14:30PM -0800, Andrew Morton wrote:
> On Tue, 19 Jan 2016 13:02:39 -0500 Johannes Weiner <hannes@...xchg.org> wrote:
> 
> > b764375 ("procfs: mark thread stack correctly in proc/<pid>/maps")
> > added [stack:TID] annotation to /proc/<pid>/maps. Finding the task of
> > a stack VMA requires walking the entire thread list, turning this into
> > quadratic behavior: a thousand threads means a thousand stacks, so the
> > rendering of /proc/<pid>/maps needs to look at a million threads. The
> > cost is not in proportion to the usefulness as described in the patch.
> > 
> > Drop the [stack:TID] annotation to make /proc/<pid>/maps (and
> > /proc/<pid>/numa_maps) usable again for higher thread counts.
> > 
> > The [stack] annotation inside /proc/<pid>/task/<tid>/maps is retained,
> > as identifying the stack VMA there is an O(1) operation.
> 
> Four years ago, ouch.
> 
> Any thoughts on the obvious back-compatibility concerns?  ie, why did
> Siddhesh implement this in the first place?  My bad for not ensuring
> that the changelog told us this.

I thought about storing the TID of the thread using the VMA as the
stack directly inside vm_area_struct; maybe using vm_private_data?
However, that's a bit of work and ugliness that I wouldn't want to
commit to until we know that people ended up using this in practice.

> I note that this patch is a partial revert - the smaps and numa_maps
> parts of b764375 remain in place.  What's up with that?

I left the stack annotations in the thread-specific files because that
sounds useful and is cheap enough - we only have to test the vma range
against that thread's stack pointer. The last changelog paragraph says
that for maps, I'll update it to include smaps and numa_maps.