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From: Thomas Gleixner <tglx@linutronix.de>
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To: Zhongqiu Han <quic_zhonhan@quicinc.com>, anna-maria@linutronix.de,
 frederic@kernel.org
Cc: linux-kernel@vger.kernel.org, quic_zhonhan@quicinc.com
Subject: Re: [PATCH] timers: Optimize get_timer_cpu_base() to reduce
 potentially redundant per_cpu_ptr() calls
In-Reply-To: <20241231150115.1978342-1-quic_zhonhan@quicinc.com>
References: <20241231150115.1978342-1-quic_zhonhan@quicinc.com>
Date: Wed, 15 Jan 2025 22:12:57 +0100
Message-ID: <87h65zbx2e.ffs@tglx>
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On Tue, Dec 31 2024 at 23:01, Zhongqiu Han wrote:
> If the timer is deferrable and NO_HZ_COMMON is enabled, the function
> get_timer_cpu_base() will call per_cpu_ptr() twice. Optimize the function
> to avoid potentially redundant per_cpu_ptr() calls.

This lacks an explanation for the second hunk which changes
get_timer_this_cpu_base().

> One of the call paths of the get_timer_cpu_base() function is through the
> lock_timer_base() function, which contains a loop. Within this loop, the
> get_timer_base() func is called, and in turn, it calls the
> get_timer_cpu_base() function. And in such a path, get_timer_cpu_base is
> a hotspot function. It is called approximately 13,000 times in 12 seconds
> on test x86 KVM machines.

Which is roughly once per millisecond and depending on the number of
CPUs that's far from a hotspot.

I'm not against the change per se, but this change log is a bit over the
top aside of ot mentioning the second hunk. I'll fix it up when
applying.

Thanks,

        tglx