linux-kernel - Re: [PATCH 1/3] LoongArch: Reduce min_delta for the arch clockevent device

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Message-ID: <CAAhV-H6vTBwi+t8cPKSo44KZKYj8ubwv2vV4FHrNH+nG=_ZXnw@mail.gmail.com>
Date: Thu, 14 Nov 2024 19:46:39 +0800
From: Huacai Chen <chenhuacai@...nel.org>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: Huacai Chen <chenhuacai@...ngson.cn>, Xuerui Wang <kernel@...0n.name>, loongarch@...ts.linux.dev, 
	Clark Williams <clrkwllms@...nel.org>, Steven Rostedt <rostedt@...dmis.org>, 
	linux-rt-devel@...ts.linux.dev, Guo Ren <guoren@...nel.org>, 
	Jiaxun Yang <jiaxun.yang@...goat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] LoongArch: Reduce min_delta for the arch clockevent device

Hi, Sebastian,

On Thu, Nov 14, 2024 at 6:21 PM Sebastian Andrzej Siewior
<bigeasy@...utronix.de> wrote:
>
> On 2024-11-08 17:15:43 [+0800], Huacai Chen wrote:
> > Now the min_delta is 0x600 (1536) for LoongArch's constant clockevent
> > device. For a 100MHz hardware timer this means ~15us. This is a little
> > big, especially for PREEMPT_RT enabled kernels. So reduce it to 1000
> > (we don't want too small values to affect performance).
>
> So this reduces it to 10us. Is anything lower than that bad performance
> wise?
Maybe I misunderstood the meaning of min_delta, but if I'm correct,
small min_delta may cause more timers to be triggered, because timers
are aligned by the granularity (min_delta). So I think min_delta
affects performance.

And I choose 10us just because I saw latency improvements when I
reduce 15us to 10us, but no more effect when I reduce it to even
lower.

Huacai



>
> > Signed-off-by: Huacai Chen <chenhuacai@...ngson.cn>
>
> Sebastian