netdev - Re: [PATCH RFC] rds: ib: Reduce the contention caused by the asynchronous workers to flush the mr pool

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Date:   Wed, 19 Jan 2022 10:49:20 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Praveen Kannoju <praveen.kannoju@...cle.com>
Cc:     Leon Romanovsky <leon@...nel.org>,
        Santosh Shilimkar <santosh.shilimkar@...cle.com>,
        "David S . Miller" <davem@...emloft.net>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "rds-devel@....oracle.com" <rds-devel@....oracle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Rama Nichanamatlu <rama.nichanamatlu@...cle.com>,
        Rajesh Sivaramasubramaniom 
        <rajesh.sivaramasubramaniom@...cle.com>
Subject: Re: [PATCH RFC] rds: ib: Reduce the contention caused by the
 asynchronous workers to flush the mr pool

On Wed, Jan 19, 2022 at 02:08:48PM +0000, Praveen Kannoju wrote:
> From: Jason Gunthorpe [mailto:jgg@...pe.ca] 
> Sent: 19 January 2022 06:47 PM
> To: Praveen Kannoju <praveen.kannoju@...cle.com>
> Cc: Leon Romanovsky <leon@...nel.org>; Santosh Shilimkar <santosh.shilimkar@...cle.com>; David S . Miller <davem@...emloft.net>; kuba@...nel.org; netdev@...r.kernel.org; linux-rdma@...r.kernel.org; rds-devel@....oracle.com; linux-kernel@...r.kernel.org; Rama Nichanamatlu <rama.nichanamatlu@...cle.com>; Rajesh Sivaramasubramaniom <rajesh.sivaramasubramaniom@...cle.com>
> Subject: Re: [PATCH RFC] rds: ib: Reduce the contention caused by the asynchronous workers to flush the mr pool
> 
> On Wed, Jan 19, 2022 at 01:12:29PM +0000, Praveen Kannoju wrote:
> 
> > Yes, we are using the barriers. I was justifying the usage of
> > smp_rmb() and smp_wmb() over smp_load_acquire() and
> > smp_store_release() in the patch.
> 
> You failed to justify it.
> 
> Jason
> 
> Apologies, if my earlier point is not clear, Jason.
> Let me reframe:
> 
> 1. The introduced bool variable "flush_ongoing", is being accessed only in the function "rds_ib_free_mr" while spawning asynchronous workers.
> 
> 2. The ordering guaranteed by smp_rmb() and smp_wmb() would be
> sufficient for such simple usage and hence we did not use
> smp_load_acquire() and smp_store_release().

Again you haven't defined why these barriers are any differnet from
acquire/release or even *what they are doing*

Jason