linux-kernel - Re: [External] Re: [PATCH v8 9/9] x86/smpboot: Serialize topology updates for secondary bringup

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Date:   Mon, 13 Feb 2023 22:30:20 +0000
From:   Usama Arif <usama.arif@...edance.com>
To:     David Woodhouse <dwmw2@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>, kim.phillips@....com
Cc:     arjan@...ux.intel.com, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, hpa@...or.com, x86@...nel.org,
        pbonzini@...hat.com, paulmck@...nel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        rcu@...r.kernel.org, mimoja@...oja.de, hewenliang4@...wei.com,
        thomas.lendacky@....com, seanjc@...gle.com, pmenzel@...gen.mpg.de,
        fam.zheng@...edance.com, punit.agrawal@...edance.com,
        simon.evans@...edance.com, liangma@...ngbit.com,
        David Woodhouse <dwmw@...zon.co.uk>
Subject: Re: [External] Re: [PATCH v8 9/9] x86/smpboot: Serialize topology
 updates for secondary bringup

On 13/02/2023 20:53, David Woodhouse wrote:
> 
> 
> On 13 February 2023 21:43:13 CET, Thomas Gleixner <tglx@...utronix.de> wrote:
>> On Thu, Feb 09 2023 at 15:41, Usama Arif wrote:
>>> From: David Woodhouse <dwmw@...zon.co.uk>
>>>
>>> The toplogy update is performed by the AP via smp_callin() after the BSP
>>> has called do_wait_cpu_initialized(), setting the AP's bit in
>>> cpu_callout_mask to allow it to proceed.
>>>
>>> In preparation to enable further parallelism of AP bringup, add locking to
>>> serialize the update even if multiple APs are (in future) permitted to
>>> proceed through the next stages of bringup in parallel.
>>
>> This one is also only relevant for further parallelisation, right?
> 
> I believe so, yes. But it's low-hanging fruit and might as well go in now.

Yes, only needed if we parallelize further, i.e. after 
do_wait_cpu_initialized. As David said, its a simple enough and easy 
patch, but its not needed for parallelizing INIT/SIPI.

I tested Davids' part2 branch 
(https://git.infradead.org/users/dwmw2/linux.git/shortlog/refs/heads/parallel-6.2-rc7) 
again just to be sure, and the only commit that makes a significant 
difference in smpboot time on top of part1 (this series) is reusing 
timer calibration (100ms to 34ms).

Parallelizing do_wait_cpu_initialized didn't significantly improve 
smpboot time (only reduced to 31ms on average of 4 runs so maybe within 
the margin of error?), so I think its better to move this patch into its 
own series with any further parallelization only if it shows a further 
improvement in smpboot time?

Thanks,
Usama