netdev - RE: [PATCH v24 03/20] iov_iter: skip copy if src == dst for direct data placement

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Message-ID: <253h6fzaxm5.fsf@nvidia.com>
Date: Thu, 18 Apr 2024 11:22:42 +0300
From: Aurelien Aptel <aaptel@...dia.com>
To: David Laight <David.Laight@...LAB.COM>, "linux-nvme@...ts.infradead.org"
 <linux-nvme@...ts.infradead.org>, "netdev@...r.kernel.org"
 <netdev@...r.kernel.org>, "sagi@...mberg.me" <sagi@...mberg.me>,
 "hch@....de" <hch@....de>, "kbusch@...nel.org" <kbusch@...nel.org>,
 "axboe@...com" <axboe@...com>, "chaitanyak@...dia.com"
 <chaitanyak@...dia.com>, "davem@...emloft.net" <davem@...emloft.net>,
 "kuba@...nel.org" <kuba@...nel.org>
Cc: "aurelien.aptel@...il.com" <aurelien.aptel@...il.com>,
 "smalin@...dia.com" <smalin@...dia.com>, "malin1024@...il.com"
 <malin1024@...il.com>, "ogerlitz@...dia.com" <ogerlitz@...dia.com>,
 "yorayz@...dia.com" <yorayz@...dia.com>, "borisp@...dia.com"
 <borisp@...dia.com>, "galshalom@...dia.com" <galshalom@...dia.com>,
 "mgurtovoy@...dia.com" <mgurtovoy@...dia.com>, "viro@...iv.linux.org.uk"
 <viro@...iv.linux.org.uk>
Subject: RE: [PATCH v24 03/20] iov_iter: skip copy if src == dst for direct
 data placement

Hi David,

David Laight <David.Laight@...LAB.COM> writes:
> How must does this conditional cost for the normal case
> when it is true?
> I suspect it is mispredicted 50% of the time.
> So, while it may speed up your test, the overall system
> impact will be negative.

We have done some measures to evaluate the cost of the additional test
and we don't see any noticeable impact on performances.

If this is still a concern, we can add compilation-time check on
CONFIG_ULP_DDP i.e. ifdef or IS_ENABLED() (which gets optimized out).

Thanks