linux-kernel - Re: [PATCH 2/2] x86/asm/delay: Introduce TPAUSE delay

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Message-ID: <20200226211040.GS160988@tassilo.jf.intel.com>
Date:   Wed, 26 Feb 2020 13:10:40 -0800
From:   Andi Kleen <ak@...ux.intel.com>
To:     Kyung Min Park <kyung.min.park@...el.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, tglx@...utronix.de,
        mingo@...hat.com, hpa@...or.com, gregkh@...uxfoundation.org,
        tony.luck@...el.com, ashok.raj@...el.com, ravi.v.shankar@...el.com,
        fenghua.yu@...el.com
Subject: Re: [PATCH 2/2] x86/asm/delay: Introduce TPAUSE delay

On Wed, Feb 26, 2020 at 11:10:58AM -0800, Kyung Min Park wrote:
> TPAUSE instructs the processor to enter an implementation-dependent
> optimized state. The instruction execution wakes up when the time-stamp
> counter reaches or exceeds the implicit EDX:EAX 64-bit input value.
> The instruction execution also wakes up due to the expiration of
> the operating system time-limit or by an external interrupt

This is actually a behavior change. Today's udelay() will continue
after processing the interrupt. Your patches don't

I don't think it's a problem though. The interrupt will cause
a long enough delay that exceed any reasonable udelay() requirements.

There would be a difference if someone did really long udelay()s, much
longer than typical interrupts, in this case you might end up
with a truncated udelay, but such long udelays are not something that we
would encourage.

I don't think you need to change anything in the code, but should
probably document this behavior.

-Andi