linux-kernel - RE: [PATCH v2] IB/mlx5: Reduce max order of memory allocated for xlt update

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Message-ID: <CY4PR10MB1448C03F77B4DDF8833489058C7B9@CY4PR10MB1448.namprd10.prod.outlook.com>
Date:   Thu, 1 Apr 2021 15:56:37 +0000
From:   Praveen Kannoju <praveen.kannoju@...cle.com>
To:     Jason Gunthorpe <jgg@...dia.com>,
        Aruna Ramakrishna <aruna.ramakrishna@...cle.com>
CC:     "leon@...nel.org" <leon@...nel.org>,
        "dledford@...hat.com" <dledford@...hat.com>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Rajesh Sivaramasubramaniom 
        <rajesh.sivaramasubramaniom@...cle.com>,
        Rama Nichanamatlu <rama.nichanamatlu@...cle.com>,
        Jeffery Yoder <jeffery.yoder@...cle.com>
Subject: RE: [PATCH v2] IB/mlx5: Reduce max order of memory allocated for xlt
 update

Hi Jason,
We will go ahead with just adding the "__GFP_NORETRY " flag to reduce the time it takes to fail the higher order memory allocations in case higher order pages are not available.

Will send out the corresponding patch.

Thank you very much for your inputs.

-
Praveen Kumar Kannoju.


-----Original Message-----
From: Jason Gunthorpe [mailto:jgg@...dia.com] 
Sent: 31 March 2021 11:23 PM
To: Aruna Ramakrishna <aruna.ramakrishna@...cle.com>
Cc: Praveen Kannoju <praveen.kannoju@...cle.com>; leon@...nel.org; dledford@...hat.com; linux-rdma@...r.kernel.org; linux-kernel@...r.kernel.org; Rajesh Sivaramasubramaniom <rajesh.sivaramasubramaniom@...cle.com>; Rama Nichanamatlu <rama.nichanamatlu@...cle.com>; Jeffery Yoder <jeffery.yoder@...cle.com>
Subject: Re: [PATCH v2] IB/mlx5: Reduce max order of memory allocated for xlt update

On Thu, Mar 25, 2021 at 11:39:28AM -0300, Jason Gunthorpe wrote:
> On Tue, Mar 23, 2021 at 09:27:38PM -0700, Aruna Ramakrishna wrote:
> 
> > > Do you have benchmarks that show the performance of the high order 
> > > pages is not relavent? I'm a bit surprised to hear that
> > > 
> > 
> > I guess my point was more to the effect that an order-8 alloc will 
> > fail more often than not, in this flow. For instance, when we were 
> > debugging the latency spikes here, this was the typical buddyinfo 
> > output on that system:
> > 
> > Node 0, zone      DMA      0      1      1      2      3      0      1      0      1      1      3 
> > Node 0, zone    DMA32      7      7      7      6     10      2      6      7      6      2    306 
> > Node 0, zone   Normal   3390  51354  17574   6556   1586     26      2      1      0      0      0 
> > Node 1, zone   Normal  11519  23315  23306   9738     73      2      0      1      0      0      0 
> > 
> > I think this level of fragmentation is pretty normal on long running 
> > systems. Here, in the reg_mr flow, the first try (order-8) alloc 
> > will probably fail 9 times out of 10 (esp. after the addition of 
> > GFP_NORETRY flag), and then as fallback, the code tries to allocate 
> > a lower order, and if that too fails, it allocates a page. I think 
> > it makes sense to just avoid trying an order-8 alloc here.
> 
> But a system like this won't get THPs either, so I'm not sure it is 
> relevant. The function was designed as it is to consume a "THP" if it 
> is available.

So can we do this with just the addition of __GFP_NORETRY ?

Jason