Re: [PATCH] mmap: avoid unnecessary anon_vma lock acquisition invma_adjust()

[Lee Schermerhorn]

On Sun, 2009-09-13 at 14:24 +0100, Hugh Dickins wrote:
> [I've kept stable@xxxxxxxxxx in the Cc list only to put on record
> that I don't think that this patch is really suitable for -stable]
> 
> On Wed, 9 Sep 2009, Lee Schermerhorn wrote:
> > 
> Thanks for this.  Interesting stuff snipped and repeated below.

Hugh:

Thank you for reviewing and correcting the patch.

Andrew: 

Shall I resend?  Or, can you grab Hugh's patch from his message?


Lee


> 
> > 
> > A comment in mm/mmap.c:vma_adjust() suggests that we don't really
> > need the anon_vma lock when we're only adjusting the end of a vma,
> 
> I feel a warm smugness from having foreseen the possibility that we
> might want to optimize that away.  I didn't do so, because it needs
> more thought (and more branches) than seemed worthwhile at the time:
> you've found similar difficulty, but now it does seem worthwhile.
> 
> > We can detect this condition--no overlap with next vma--by noting
> > a NULL "importer".  The anon_vma pointer will also be NULL in this
> > case, so simply avoid loading vma->anon_vma to avoid the lock.
> > However, we apparently DO need to take the anon_vma lock when
> > we're inserting a vma ['insert' non-NULL] even when we have no
> > overlap [NULL "importer"], so we need to check for 'insert', as well.
> 
> Those importer and insert checks are good and relevant, but not
> quite enough.  The anon_vma lock should also be guaranteeing the
> integrity of the relationship between vm_start and vm_pgoff for
> all the vmas attached to the anon_vma, so that rmap.c can rely
> upon vma_address() to work correctly while it holds anon_vma lock.
> 
> That's a considerably less important consideration than the integrity
> of the list threading itself.  Anything BUGging on a wrong page->index
> is holding mmap_sem, which would keep vma_adjust off.  So it's just a
> matter of whether rmap.c can be expected to find all instances of a
> page at all times, which nothing absolutely requires (and in checking
> this patch, I notice fs/exec.c's shift_arg_pages() use of vma_adjust()
> a little violatory in that respect).
> 
> But it is something the anon_vma lock has protected in the past,
> and it shouldn't affect your sbrk() case at all, so I'd like to
> check we're not changing vm_start too (vm_pgoff should be changing
> with it, but shift_arg_pages() deals with that in a different way,
> keeping vm_pgoff unchanged but shifting the pages).
> 
> (Compare with how stack's expand_downwards() has anon_vma_lock()
> when it adjusts vm_start and vm_pgoff - though that's also because
> it has only down_read of mmap_sem, not the down_write we'd usually
> require for such adjustments.)
> 
> > 
> > I have tested with and without the 'file || ' test in the patch.
> > This does not seem to matter for stability nor performance.  I
> > left this check/filter in, so we only optimize away the
> > anon_vma lock acquisition when adjusting the end of a non-
> > importing, non-inserting, anon vma.
> 
> I dislike that: the "file" test just has no relevance at all
> (beyond that you've no interest in the case when there is a file),
> and two years down the line will make people like me worry for
> hours on end what it's there for.  I've removed it below.
> 
> > 
> > If accepted, this patch may benefit the stable tree as well.
> 
> We seem to have a different perception of what the stable tree is for!
> But I'm not against any distro picking up this patch if it chooses.
> Here's a version with my signoff:
> 
> 
> [PATCH] mmap: avoid unnecessary anon_vma lock acquisition in vma_adjust()
> 
> From: Lee Schermerhorn <lee.schermerhorn@xxxxxx>
> 
> We noticed very erratic behavior [throughput] with the AIM7 shared
> workload running on recent distro [SLES11] and mainline kernels on
> an 8-socket, 32-core, 256GB x86_64 platform.  On the SLES11 kernel
> [2.6.27.19+] with Barcelona processors, as we increased the load
> [10s of thousands of tasks], the throughput would vary between two
> "plateaus"--one at ~65K jobs per minute and one at ~130K jpm.  The
> simple patch below causes the results to smooth out at the ~130k
> plateau.
> 
> But wait, there's more:
> 
> We do not see this behavior on smaller platforms--e.g., 4 socket/8
> core.  This could be the result of the larger number of cpus on
> the larger platform--a scalability issue--or it could be the result
> of the larger number of interconnect "hops" between some nodes in
> this platform and how the tasks for a given load end up distributed
> over the nodes' cpus and memories--a stochastic NUMA effect.
> 
> The variability in the results are less pronounced [on the same
> platform] with Shanghai processors and with mainline kernels.  With
> 31-rc6 on Shanghai processors and 288 file systems on 288 fibre
> attached storage volumes, the curves [jpm vs load] are both quite
> flat with the patched kernel consistently producing ~3.9% better
> throughput [~80K jpm vs ~77K jpm] than the unpatched kernel.
> 
> Profiling indicated that the "slow" runs were incurring high[er]
> contention on an anon_vma lock in vma_adjust(), apparently called
> from the sbrk() system call.
> 
> The patch:
> 
> A comment in mm/mmap.c:vma_adjust() suggests that we don't really
> need the anon_vma lock when we're only adjusting the end of a vma,
> as is the case for brk().  The comment questions whether it's worth
> while to optimize for this case.  Apparently, on the newer, larger
> x86_64 platforms, with interesting NUMA topologies, it is worth
> while--especially considering that the patch [if correct!] is 
> quite simple.
> 
> We can detect this condition--no overlap with next vma--by noting
> a NULL "importer".  The anon_vma pointer will also be NULL in this
> case, so simply avoid loading vma->anon_vma to avoid the lock.
> 
> However, we DO need to take the anon_vma lock when we're inserting a
> vma ['insert' non-NULL] even when we have no overlap [NULL "importer"],
> so we need to check for 'insert', as well.  And Hugh points out that
> we should also take it when adjusting vm_start (so that rmap.c can
> rely upon vma_address() while it holds the anon_vma lock).
> 
> Signed-off-by: Lee Schermerhorn <lee.schermerhorn@xxxxxx>
> Signed-off-by: Hugh Dickins <hugh.dickins@xxxxxxxxxxxxx>
> Cc: Nick Piggin <npiggin@xxxxxxx>
> Cc: Eric Whitney <eric.whitney@xxxxxx>
> ---
> 
>  mm/mmap.c |    4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> --- 2.6.31/mm/mmap.c	2009-09-09 23:13:59.000000000 +0100
> +++ linux/mm/mmap.c	2009-09-13 13:08:40.000000000 +0100
> @@ -570,9 +570,9 @@ again:			remove_next = 1 + (end > next->
>  
>  	/*
>  	 * When changing only vma->vm_end, we don't really need
> -	 * anon_vma lock: but is that case worth optimizing out?
> +	 * anon_vma lock.
>  	 */
> -	if (vma->anon_vma)
> +	if (vma->anon_vma && (insert || importer || start != vma->vm_start))
>  		anon_vma = vma->anon_vma;
>  	if (anon_vma) {
>  		spin_lock(&anon_vma->lock);
> 
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