netdev - Re: 2.6.21 -> 2.6.22 & 2.6.23-rc8 performance regression

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Message-Id: <20071001.001259.28812610.davem@davemloft.net>
Date:	Mon, 01 Oct 2007 00:12:59 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	dada1@...mosbay.com
Cc:	nuclearcat@...learcat.com, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org
Subject: Re: 2.6.21 -> 2.6.22 & 2.6.23-rc8 performance regression

From: Eric Dumazet <dada1@...mosbay.com>
Date: Mon, 01 Oct 2007 07:59:12 +0200

> No problem here on bigger servers, so I CC David Miller and netdev
> on this one.  AFAIK do_gettimeofday() and ktime_get_real() should
> use the same underlying hardware functions on PC and no performance
> problem should happen here.

One thing that jumps out at me is that on 32-bit (and to a certain
extent on 64-bit) there is a lot of stack accesses and missed
optimizations because all of the work occurs, and gets expanded,
inside of ktime_get_real().

The timespec_to_ktime() inside of there constructs the ktime_t return
value on the stack, then returns that as an aggregate to the caller.

That cannot be without some cost.

ktime_get_real() is definitely a candidate for inlining especially in
these kinds of cases where we'll happily get computations in local
registers instead of all of this on-stack nonsense.  And in several
cases (if the caller only needs the tv_sec value, for example)
computations can be elided entirely.

It would be constructive to experiment and see if this is in fact part
of the problem.

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