linux-kernel - Re: cli/sti vs local_cmpxchg and local_add

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Message-Id: <20090316.203705.218202510.davem@davemloft.net>
Date:	Mon, 16 Mar 2009 20:37:05 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	mathieu.desnoyers@...ymtl.ca
Cc:	paulmck@...ux.vnet.ibm.com, mingo@...e.hu,
	jwboyer@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
	ltt-dev@...ts.casi.polymtl.ca
Subject: Re: cli/sti vs local_cmpxchg and local_add_return

From: Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Date: Mon, 16 Mar 2009 21:32:20 -0400

> If some of you would be kind enough to run my test module provided below
> and provide the results of these tests on a recent kernel (2.6.26~2.6.29
> should be good) along with their cpuinfo, I would greatly appreciate.

Here's sparc64, but cycles is always computed as zero.

Probably that's because get_cycles() on sparc64 counts system
bus clock cycles, not CPU cycles, and you the loop iteration
isn't expensive enough to get a system clock tick in.

This is a dual UltraSPARC-IIIi at 1.2 GHz

[1052598.484452] test init
[1052598.486230] test results: time for baseline
[1052598.487878] number of loops: 20000
[1052598.489485] total time: 752
[1052598.491061] -> baseline takes 0 cycles
[1052598.492649] test end
[1052598.494874] test results: time for locked cmpxchg
[1052598.496460] number of loops: 20000
[1052598.498005] total time: 7879
[1052598.499521] -> locked cmpxchg takes 0 cycles
[1052598.501060] test end
[1052598.503194] test results: time for non locked cmpxchg
[1052598.504733] number of loops: 20000
[1052598.506213] total time: 7879
[1052598.507722] -> non locked cmpxchg takes 0 cycles
[1052598.509229] test end
[1052598.511347] test results: time for locked add return
[1052598.512821] number of loops: 20000
[1052598.514265] total time: 8254
[1052598.515682] -> locked add return takes 0 cycles
[1052598.517130] test end
[1052598.519427] test results: time for non locked add return
[1052598.520850] number of loops: 20000
[1052598.522230] total time: 11259
[1052598.523561] -> non locked add return takes 0 cycles
[1052598.524939] test end
[1052598.526393] test results: time for enabling interrupts (STI)
[1052598.527767] number of loops: 20000
[1052598.529085] total time: 1877
[1052598.530373] -> enabling interrupts (STI) takes 0 cycles
[1052598.531713] test end
[1052598.533240] test results: time for disabling interrupts (CLI)
[1052598.534594] number of loops: 20000
[1052598.535892] total time: 3189
[1052598.537189] -> disabling interrupts (CLI) takes 0 cycles
[1052598.538551] test end
[1052598.540176] test results: time for disabling/enabling interrupts (STI/CLI)
[1052598.541579] number of loops: 20000
[1052598.542900] total time: 3940
[1052598.544207] -> enabling/disabling interrupts (STI/CLI) takes 0 cycles
[1052598.545595] test end

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