linux-kernel - Re: [RFC PATCH] hrtimer: interleave timers for improved single thread performance at low utilization

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Message-ID: <c6c106a2-b180-d3fd-5904-44f9b4949ddc@linux.intel.com>
Date:   Tue, 31 Jan 2023 06:55:47 -0800
From:   Arjan van de Ven <arjan@...ux.intel.com>
To:     shrikanth hegde <sshegde@...ux.vnet.ibm.com>, tglx@...utronix.de
Cc:     peterz@...radead.org, mingo@...nel.org,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        svaidy@...ux.ibm.com, linux-kernel@...r.kernel.org,
        bigeasy@...utronix.de
Subject: Re: [RFC PATCH] hrtimer: interleave timers for improved single thread
 performance at low utilization


>   kernel/time/hrtimer.c | 11 +++++++++++
>   1 file changed, 11 insertions(+)
> 
> diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
> index 3ae661ab6260..d160f49f0cce 100644
> --- a/kernel/time/hrtimer.c
> +++ b/kernel/time/hrtimer.c
> @@ -1055,6 +1055,17 @@ u64 hrtimer_forward(struct hrtimer *timer, ktime_t now, ktime_t interval)
> 
>           orun = ktime_divns(delta, incr);
>           hrtimer_add_expires_ns(timer, incr * orun);
> +        /*
> +         * Avoid timer round-off, so that all cfs bandwidth timers
> +         * don't start at the same time

so while I applaud the final objective, I am sort of wondering if hrtimer.c is the right place in the kernel to fix a CFS/cgroup issue...
wouldn't it be better to solve such issues at the place we want this to happen, rather than for all timers in the whole system?

(also while for performance it might be better to spread out a bit, for power consumption it's obviously the other way around)