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

[Usama Arif]

On 13/02/2023 20:53, David Woodhouse wrote:
> On 13 February 2023 21:43:13 CET, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> > On Thu, Feb 09 2023 at 15:41, Usama Arif wrote:
> > > From: David Woodhouse <dwmw@xxxxxxxxxxxx>
> > >
> > > 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