linux-kernel - Re: [PATCH 1/5] x86/intel_rdt: Update control registers only when user really modifies it

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Message-ID: <alpine.DEB.2.10.1703091534380.5245@vshiva-Udesk>
Date:   Thu, 9 Mar 2017 16:00:48 -0800 (PST)
From:   Shivappa Vikas <vikas.shivappa@...ux.intel.com>
To:     Thomas Gleixner <tglx@...utronix.de>
cc:     Vikas Shivappa <vikas.shivappa@...ux.intel.com>,
        vikas.shivappa@...el.com, linux-kernel@...r.kernel.org,
        x86@...nel.org, hpa@...or.com, mingo@...nel.org,
        peterz@...radead.org, ravi.v.shankar@...el.com,
        tony.luck@...el.com, fenghua.yu@...el.com, andi.kleen@...el.com
Subject: Re: [PATCH 1/5] x86/intel_rdt: Update control registers only when
 user really modifies it



On Wed, 1 Mar 2017, Thomas Gleixner wrote:

> On Fri, 17 Feb 2017, Vikas Shivappa wrote:
>
> x86/intel_rdt: Update control registers only when user really modifies it
>
> This hardly is a precise short summary.
>
>> When a schemata is updated, the values for all the domains and all
>> resources are entered.  However, the values for each of them may not
>> change in many use cases as the user is only updating values for a
>> subset of resources and domains. The resource control values are updated
>> via QOS_MSRs which are per package. Change the update to QOS_MSRs to
>> happen only when the control value on the particular domain is updated.
>> Hence not sending IPIs on all domains when user updates the control
>> vals.
>
> Can you please structure your changelogs in a way which makes them
> readable? The above is one big confusing lump. I asked you before to
> provide changelogs which are properly structured:
>
> 1) Context
> 2) Problem
> 3) Solution

Will fix all the change logs -

>
> and the sections to be precise and clear and not clobbered with completely
> useless implementation details.
>
> So a proper changelog for this would be:
>
> x86/intel_rdt: Avoid update of unchanged control registers
>
>   Schemata files can only be updated as a whole, even if only a single
>   value for a specific domain/resource changes.
>
>   The current implementation updates all control registers unconditionally
>   even if the values have not been changed by the schemata update. This
>   results in pointless IPIs and MSR writes.
>
>   Add a check whether the control register value actually changed and only
>   update the affected CPUs.
>
> Can you spot the difference?
>
>> index f369cb8..14ba504 100644
>> --- a/arch/x86/kernel/cpu/intel_rdt_schemata.c
>> +++ b/arch/x86/kernel/cpu/intel_rdt_schemata.c
>> @@ -114,9 +114,16 @@ static int update_domains(struct rdt_resource *r, int closid)
>>  	msr_param.high = msr_param.low + 1;
>>  	msr_param.res = r;
>>
>> +	/*
>> +	 * Only update the domains that user has changed.
>> +	 * There by avoiding unnecessary IPIs.
>
> s/There by/Thereby/
>
> But the above is wrong anyway because you split it into two sentences and
> obfuscate the reasoning.
>
> 	 * To avoid unnecessary IPIs update only domains, which have been
>         * changed by the schemata write.
>
> That makes it clear that we do it in order to avoid the IPIs. The above
> could be misinterpreted as having the side effect of avoiding the IPIs.
>

Will fix the comment

Thanks,
Vikas

> Thanks,
>
> 	tglx
>