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 13:12:29 +0000
From:   Praveen Kannoju <praveen.kannoju@...cle.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
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



-----Original Message-----
From: Jason Gunthorpe [mailto:jgg@...pe.ca] 
Sent: 19 January 2022 06:35 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 11:46:16AM +0000, Praveen Kannoju wrote:
 
> 6. Jason, the only function "rds_ib_free_mr" which accesses the 
> introduced bool variable "flush_ongoing" to spawn a flush worker does 
> not crucially impact the availability of MR's, because the flush 
> happens from allocation path as well when necessary.  Hence the 
> Load-store ordering is not essentially needed here, because of which 
> we chose smp_rmb() and smp_wmb() over smp_load_acquire() and 
> smp_store_release().

That seems like a confusing statement, you added barriers which do the same things as acquire/release then say you didn't need acquire release?

I think this is using barriers wrong.

Jason

Jason, 

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.