Re: [PATCH v6 for Xen 4.7 1/4] xen: enable per-VCPU parameter settings for RTDS scheduler

From: Chong Li <lichong659@gmail.com>
To: Dario Faggioli <dario.faggioli@citrix.com>
Cc: Chong Li <chong.li@wustl.edu>, Wei Liu <wei.liu2@citrix.com>,
	Sisu Xi <xisisu@gmail.com>,
	GeorgeDunlap <george.dunlap@eu.citrix.com>,
	xen-devel <xen-devel@lists.xen.org>,
	Meng Xu <mengxu@cis.upenn.edu>, Jan Beulich <JBeulich@suse.com>,
	Dagaen Golomb <dgolomb@seas.upenn.edu>
Subject: Re: [PATCH v6 for Xen 4.7 1/4] xen: enable per-VCPU parameter settings for RTDS scheduler
Date: Tue, 15 Mar 2016 11:22:06 -0500	[thread overview]
Message-ID: <CAGHO-irYHPdH3c65reUmDNv-hQ51UX_4pPLUFR_KmfD1jpPxOw@mail.gmail.com> (raw)
In-Reply-To: <1457949930.3102.641.camel@citrix.com>

On Mon, Mar 14, 2016 at 5:05 AM, Dario Faggioli
<dario.faggioli@citrix.com> wrote:
> On Mon, 2016-03-14 at 03:15 -0600, Jan Beulich wrote:
>> > > > On 14.03.16 at 10:10, <dario.faggioli@citrix.com> wrote:
>> >
>> > I think it has some value to have this in the logs. In fact,
>> > someone
>> > that ended up with small values --either by bug/chance, or in
>> > general
>> > without a specific need for them-- and is seeing
>> > performance/scheduling
>> > issues will still be able to find this.
>> >
>> > And printing it somewhere else than in Xen is impractical (see my
>> > reply
>> > to Wei).
>> >
>> > However, we may well print it just once, as soon as the first vcpu
>> > with
>> > potentially problematic parameters is hit, and then silence it.
>> >
>> > Linux has things like WARN_ON_ONCE to do this:
>> > http://lxr.free-electrons.com/source/include/asm-generic/bug.h#L109
>> >
>> >
>> > I don't see anything like it in Xen, so you'd need to either
>> > implement
>> > it, or arrange for something with the same effect locally.
>> One of the reasons we don't (yet) have this is because commonly
>> (like is the case here) printing such once globally isn't really what
>> you want - you'd much rather see it once per domain.
>>
> Ah, I did not know that was part of the reason, but it makes sense.
>
> However, for this specific case, I was also considering that, and I
> agree, if possible/not to difficult to implement, once per domain would
> indeed be better.
>
It is easy to implement a global "warn_on_once".

To implement a per-domain "warn_on_once", I may need a hash table
(name it as "dom_warned"), and dom_warned[domid] returns true or
false.
I don't know if hash table is supported in Xen. Or do we have any list
structure which is very good at searching?

Another way is to add a "bool" field in struct domain, but I don't
think that would be a good design.

Any ideas?

Thanks,
Chong

-- 
Chong Li
Department of Computer Science and Engineering
Washington University in St.louis

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