Re: [PATCH v4 0/9] Parallel CPU bringup for x86_64

[David Woodhouse]

On Wed, 2023-02-01 at 14:40 +0000, Usama Arif wrote:
> On 01/02/2022 20:53, David Woodhouse wrote:
> > Doing the INIT/SIPI/SIPI in parallel for all APs and *then* waiting for
> > them shaves about 80% off the AP bringup time on a 96-thread 2-socket
> > Skylake box (EC2 c5.metal) — from about 500ms to 100ms.
> > 
> > There are more wins to be had with further parallelisation, but this is
> > the simple part.
> > 
> 
> Hi,
> 
> We are interested in reducing the boot time of servers (with kexec), and 
> smpboot takes up a significant amount of time while booting. When 
> testing the patch series (rebased to v6.1) on a server with 128 CPUs 
> split across 2 NUMA nodes, it brought down the smpboot time from ~700ms 
> to 100ms. Adding another cpuhp state for do_wait_cpu_initialized to make 
> sure cpu_init is reached (as done in v1 of the series + using the 
> cpu_finishup_mask) brought it down further to ~30ms.
> 
> I just wanted to check what was needed to progress the patch series 
> further for review? There weren't any comments on v4 of the patch so I 
> couldn't figure out what more is needed. I think its quite useful to 
> have this working so would be really glad help in anything needed to 
> restart the review.


I believe the only thing holding it back was the fact that it broke on
some AMD CPUs.

We don't *think* there are any remaining software issues; we think it's
hardware. Either an actual hardware race in CPU or chipset, or perhaps
even something as simple as a voltage regulator which can't cope with
an increase in power draw from *all* the CPUs at the same time.

We have prodded AMD a few times to investigate, but so far to no avail.

Last time I actually spoke to Thomas in person, I think he agreed that
we should just merge it and disable the parallel mode for the affected
AMD CPUs.

If you've already rebased to a newer kernel and tested it, perhaps now
is the time to do just that.

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