linux-kernel - Re: [PATCH] sched/rt: overrun could happen in start_hrtick

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Message-ID: <20140708074943.GK19379@twins.programming.kicks-ass.net>
Date:	Tue, 8 Jul 2014 09:49:43 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"xiaofeng.yan" <xiaofeng.yan@...wei.com>
Cc:	Li Zefan <lizefan@...wei.com>, mingo@...hat.com,
	linux-kernel@...r.kernel.org, xiaofeng.yan2012@...il.com,
	Juri Lelli <juri.lelli@...il.com>
Subject: Re: [PATCH] sched/rt: overrun could happen in start_hrtick_dl

On Tue, Jul 08, 2014 at 10:51:02AM +0800, xiaofeng.yan wrote:
> On 2014/7/8 10:40, Li Zefan wrote:
> >On 2014/7/8 9:10, xiaofeng.yan wrote:
> >>On 2014/7/7 16:41, Peter Zijlstra wrote:
> >>>On Fri, Jul 04, 2014 at 12:02:21PM +0000, xiaofeng.yan wrote:
> >>>>It could be wrong for the precision of runtime and deadline
> >>>>when the precision is within microsecond level. For example:
> >>>>Task runtime deadline period
> >>>>   P1   200us   500us   500us
> >>>>
> >>>>This case need enbale HRTICK feature by the next command
> >>>>PC#echo "HRTICK" > /sys/kernel/debug/sched_features
> >>>>PC#./schedtool -E -t 200000:500000 -e ./test&
> >>>>PC#trace-cmd record -e sched_switch
> >>>Are you actually using HRTICK ?
> >>yes, If HRTICK is close , then all of runtime and deadline will be wrong.
> >I think what peter meant is, do you use HRTICK in products or
> >just use it for testing/experiment?
> >
> Thanks for your timely comments. In fact, We use HRTICK feature in product.
> We need microsecond level
> precision.

Ah, thanks. Be advised that currently HRTICK is rather expensive. The
cost is twofold: 1) doing all the kernel side hrtimer things and 2)
programming clock hardware.

Of course, if that's what you need, you're willing to pay the price.

I'll see if I can put making it less expensive slightly higher on the
(endless) todo list.

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