linux-kernel - Re: [PATCH 1/2] kernel/latencytop: Add non-scheduler interface for latency reporting

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Message-ID: <20151201154655.GD26256@nuc-i3427.alporthouse.com>
Date:	Tue, 1 Dec 2015 15:46:55 +0000
From:	Chris Wilson <chris@...is-wilson.co.uk>
To:	Daniel Vetter <daniel.vetter@...ll.ch>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	DRI Development <dri-devel@...ts.freedesktop.org>,
	Intel Graphics Development <intel-gfx@...ts.freedesktop.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Daniel Vetter <daniel.vetter@...el.com>
Subject: Re: [PATCH 1/2] kernel/latencytop: Add non-scheduler interface for
 latency reporting

On Tue, Dec 01, 2015 at 04:29:27PM +0100, Daniel Vetter wrote:
> Some sources of significant amounts of latency aren't simple sleeps
> but instead busy-loops or a series of hundreds of small sleeps simply
> because the hardware can't do better. Unfortunately latencytop doesn't
> register these and so they slip under the radar. Hence expose a
> simplified interface to report additional latencies and export the
> underlying function so that modules can use this.
> 
> The example I have in mind are edid reads. The drm subsystem exposes
> both interfaces to do full probes and to just get at the cached state
> from the last probe and often userspace developers don't know about
> the difference and incur unecessary big latencies. And usually the i2c
> transfer is done with busy-looping or if there is a hw engine it might
> only be able to transfer a few bytes per sleep/irq cycle. And edid
> reads take at least 12ms and with crappy hw can easily be a few
> hundred ms.
> 
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Arjan van de Ven <arjan@...ux.intel.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Signed-off-by: Daniel Vetter <daniel.vetter@...el.com>
> ---
>  include/linux/latencytop.h | 15 +++++++++++++++
>  kernel/latencytop.c        |  2 ++
>  2 files changed, 17 insertions(+)
> 
> diff --git a/include/linux/latencytop.h b/include/linux/latencytop.h
> index e23121f9d82a..46b69bc35f02 100644
> --- a/include/linux/latencytop.h
> +++ b/include/linux/latencytop.h
> @@ -10,6 +10,9 @@
>  #define _INCLUDE_GUARD_LATENCYTOP_H_
>  
>  #include <linux/compiler.h>
> +
> +#include <asm/current.h>
> +
>  struct task_struct;
>  
>  #ifdef CONFIG_LATENCYTOP
> @@ -35,6 +38,13 @@ account_scheduler_latency(struct task_struct *task, int usecs, int inter)
>  		__account_scheduler_latency(task, usecs, inter);
>  }
>  
> +static inline void
> +account_latency(int usecs)
> +{
> +	if (unlikely(latencytop_enabled))
> +		__account_scheduler_latency(current, usecs, 0);

Just

	account_scheduler_latency(current, usecs, 0);
> +}

And then that can be used for both ifdef paths, i.e. move account_latency() to after the #endif.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre
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