Re: [RFC PATCH v1 1/6] sched: track time spent in IRQ handler

From: Julien Grall <julien@xen.org>
To: Volodymyr Babchuk <Volodymyr_Babchuk@epam.com>,
	"jgross@suse.com" <jgross@suse.com>,
	"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>
Cc: "sstabellini@kernel.org" <sstabellini@kernel.org>,
	"wl@xen.org" <wl@xen.org>,
	"andrew.cooper3@citrix.com" <andrew.cooper3@citrix.com>,
	"ian.jackson@eu.citrix.com" <ian.jackson@eu.citrix.com>,
	"george.dunlap@citrix.com" <george.dunlap@citrix.com>,
	"dfaggioli@suse.com" <dfaggioli@suse.com>,
	"jbeulich@suse.com" <jbeulich@suse.com>,
	"roger.pau@citrix.com" <roger.pau@citrix.com>
Subject: Re: [RFC PATCH v1 1/6] sched: track time spent in IRQ handler
Date: Fri, 12 Jun 2020 12:29:56 +0100	[thread overview]
Message-ID: <fcedf156-4ed6-c56a-482d-df2f867f7b3e@xen.org> (raw)
In-Reply-To: <7ec7b6568afb3df41f8407015c198b1ccb341c5b.camel@epam.com>

On 12/06/2020 12:26, Volodymyr Babchuk wrote:
> Hi Jurgen,
> 
> thanks for the review
> 
> On Fri, 2020-06-12 at 06:36 +0200, Jürgen Groß wrote:
> 
>> On 12.06.20 02:22, Volodymyr Babchuk wrote:
> 
> [...]
> 
>>> +void vcpu_end_irq_handler(void)
>>> +{
>>> +    int delta;
>>> +
>>> +    if (is_idle_vcpu(current))
>>> +        return;
>>> +
>>> +    ASSERT(current->irq_nesting);
>>> +
>>> +    if ( --current->irq_nesting )
>>> +        return;
>>> +
>>> +    /* We assume that irq handling time will not overflow int */
>>
>> This assumption might not hold for long running VMs.
> 
> Basically, this value holds time span between calls to schedule(). This
> variable gets zeroed out every time scheduler requests for time
> adjustment value. So, it should not depend on total VM run time.
This is assuming that the scheduler will be called. With the NULL 
scheduler in place, there is a fair chance this may never be called.

So I think using a 64-bit value is likely safer.

Cheers,

-- 
Julien Grall