linux-kernel - Re: [PATCH] x86/xen: remove unneeded preempt_disable() from xen_irq

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Date:   Tue, 21 Sep 2021 09:58:28 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Jan Beulich <jbeulich@...e.com>
Cc:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, xen-devel@...ts.xenproject.org,
        x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/xen: remove unneeded preempt_disable() from
 xen_irq_enable()

On 21.09.21 09:53, Jan Beulich wrote:
> On 21.09.2021 09:02, Juergen Gross wrote:
>> --- a/arch/x86/xen/irq.c
>> +++ b/arch/x86/xen/irq.c
>> @@ -57,24 +57,20 @@ asmlinkage __visible void xen_irq_enable(void)
>>   {
>>   	struct vcpu_info *vcpu;
>>   
>> -	/*
>> -	 * We may be preempted as soon as vcpu->evtchn_upcall_mask is
>> -	 * cleared, so disable preemption to ensure we check for
>> -	 * events on the VCPU we are still running on.
>> -	 */
>> -	preempt_disable();
>> -
>>   	vcpu = this_cpu_read(xen_vcpu);
>>   	vcpu->evtchn_upcall_mask = 0;
>>   
>> -	/* Doesn't matter if we get preempted here, because any
>> -	   pending event will get dealt with anyway. */
>> +	/*
>> +	 * Now preemption could happen, but this is only possible if an event
>> +	 * was handled, so missing an event due to preemption is not
>> +	 * possible at all.
>> +	 * The worst possible case is to be preempted and then check events
>> +	 * pending on the old vcpu, but this is not problematic.
>> +	 */
> 
> I agree this isn't problematic from a functional perspective, but ...
> 
>>   	barrier(); /* unmask then check (avoid races) */
>>   	if (unlikely(vcpu->evtchn_upcall_pending))
>>   		xen_force_evtchn_callback();
> 
> ... is a stray call here cheaper than ...
> 
>> -
>> -	preempt_enable();
> 
> ... the preempt_{dis,en}able() pair?

The question is if a stray call in case of preemption (very unlikely)
is cheaper than the preempt_{dis|en}able() pair on each IRQ enabling.

I'm quite sure removing the preempt_*() calls will be a net benefit.


Juergen

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