Re: [PATCH V4] mlx4_core: allocate ICM memory in page size chunks

[Qing Huang]

On 6/4/2018 5:40 AM, Vlastimil Babka wrote:
> On 06/04/2018 08:27 AM, Michal Hocko wrote:
> > On Fri 01-06-18 15:05:26, Qing Huang wrote:
> > > On 6/1/2018 12:31 AM, Michal Hocko wrote:
> > > > On Thu 31-05-18 19:04:46, Qing Huang wrote:
> > > > > On 5/31/2018 2:10 AM, Michal Hocko wrote:
> > > > > > On Thu 31-05-18 10:55:32, Michal Hocko wrote:
> > > > > > > On Thu 31-05-18 04:35:31, Eric Dumazet wrote:
> > > > > > [...]
> > > > > > > > I merely copied/pasted from alloc_skb_with_frags() :/
> > > > > > > I will have a look at it. Thanks!
> > > > > > OK, so this is an example of an incremental development ;).
> > > > > >
> > > > > > __GFP_NORETRY was added by ed98df3361f0 ("net: use __GFP_NORETRY for
> > > > > > high order allocations") to prevent from OOM killer. Yet this was
> > > > > > not enough because fb05e7a89f50 ("net: don't wait for order-3 page
> > > > > > allocation") didn't want an excessive reclaim for non-costly orders
> > > > > > so it made it completely NOWAIT while it preserved __GFP_NORETRY in
> > > > > > place which is now redundant. Should I send a patch?
> > > > > Just curious, how about GFP_ATOMIC flag? Would it work in a similar fashion?
> > > > > We experimented
> > > > > with it a bit in the past but it seemed to cause other issue in our tests.
> > > > > :-)
> > > > GFP_ATOMIC is a non-sleeping (aka no reclaim) context with an access to
> > > > memory reserves. So the risk is that you deplete those reserves and
> > > > cause issues to other subsystems which need them as well.
> > > >
> > > > > By the way, we didn't encounter any OOM killer events. It seemed that the
> > > > > mlx4_alloc_icm() triggered slowpath.
> > > > > We still had about 2GB free memory while it was highly fragmented.
> > > > The compaction was able to make a reasonable forward progress for you.
> > > > But considering mlx4_alloc_icm is called with GFP_KERNEL resp. GFP_HIGHUSER
> > > > then the OOM killer is clearly possible as long as the order is lower
> > > > than 4.
> > > The allocation was 256KB so the order was much higher than 4. The compaction
> > > seemed to be the root
> > > cause for our problem. It took too long to finish its work while putting
> > > mlx4_alloc_icm to sleep in a heavily
> > > fragmented memory situation . Will NORETRY flag avoid the compaction ops and
> > > fail the 256KB allocation
> > > immediately so mlx4_alloc_icm can enter adjustable lower order allocation
> > > code path quickly?
> > Costly orders should only perform a light compaction attempt unless
> > __GFP_RETRY_MAY_FAIL is used IIRC. CCing Vlastimil. So __GFP_NORETRY
> > shouldn't make any difference.
> It's a bit more complicated. Costly allocations will try the light
> compaction attempt first, even before reclaim. This is followed by
> reclaim and a more costly compaction attempt. With __GFP_NORETRY, the
> second compaction attempt is also only the light one, so the flag does
> make a difference here.

Thanks for the clarification!

Looks like our production kernel is kinda old, neither 
__GFP_DIRECT_RECLAIM nor __GFP_NORETRY
has been used in __alloc_pages_slowpath() in our kernel.