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@xxxxxxxxxxxxx>
> Cc: Arjan van de Ven <arjan@xxxxxxxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx>
> ---
> 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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