linux-kernel - Re: [PATCH RFC v3 1/6] x86/paravirt: Add pv_idle

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Date:   Mon, 13 Nov 2017 11:53:52 +0100
From:   Juergen Gross <jgross@...e.com>
To:     Quan Xu <quan.xu03@...il.com>, kvm@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, x86@...nel.org,
        xen-devel@...ts.xenproject.org
Cc:     Quan Xu <quan.xu0@...il.com>, Yang Zhang <yang.zhang.wz@...il.com>,
        Alok Kataria <akataria@...are.com>,
        Rusty Russell <rusty@...tcorp.com.au>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH RFC v3 1/6] x86/paravirt: Add pv_idle_ops to paravirt ops

On 13/11/17 11:06, Quan Xu wrote:
> From: Quan Xu <quan.xu0@...il.com>
> 
> So far, pv_idle_ops.poll is the only ops for pv_idle. .poll is called
> in idle path which will poll for a while before we enter the real idle
> state.
> 
> In virtualization, idle path includes several heavy operations
> includes timer access(LAPIC timer or TSC deadline timer) which will
> hurt performance especially for latency intensive workload like message
> passing task. The cost is mainly from the vmexit which is a hardware
> context switch between virtual machine and hypervisor. Our solution is
> to poll for a while and do not enter real idle path if we can get the
> schedule event during polling.
> 
> Poll may cause the CPU waste so we adopt a smart polling mechanism to
> reduce the useless poll.
> 
> Signed-off-by: Yang Zhang <yang.zhang.wz@...il.com>
> Signed-off-by: Quan Xu <quan.xu0@...il.com>
> Cc: Juergen Gross <jgross@...e.com>
> Cc: Alok Kataria <akataria@...are.com>
> Cc: Rusty Russell <rusty@...tcorp.com.au>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: "H. Peter Anvin" <hpa@...or.com>
> Cc: x86@...nel.org
> Cc: virtualization@...ts.linux-foundation.org
> Cc: linux-kernel@...r.kernel.org
> Cc: xen-devel@...ts.xenproject.org

Hmm, is the idle entry path really so critical to performance that a new
pvops function is necessary? Wouldn't a function pointer, maybe guarded
by a static key, be enough? A further advantage would be that this would
work on other architectures, too.


Juergen