linux-kernel - Re: [PATCH v2] iov_iter: optimize page_copy

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Message-ID: <20190226190431.GK2217@ZenIV.linux.org.uk>
Date:   Tue, 26 Feb 2019 19:04:32 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH v2] iov_iter: optimize page_copy_sane()

On Tue, Feb 26, 2019 at 10:42:39AM -0800, Eric Dumazet wrote:
> Avoid cache line miss dereferencing struct page if we can.
> 
> page_copy_sane() mostly deals with order-0 pages.
> 
> Extra cache line miss is visible on TCP recvmsg() calls dealing
> with GRO packets (typically 45 page frags are attached to one skb).
> 
> Bringing the 45 struct pages into cpu cache while copying the data
> is not free, since the freeing of the skb (and associated
> page frags put_page()) can happen after cache lines have been evicted.

Applied.