From: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
To: David Gibson <david@gibson.dropbear.id.au>
Cc: Nathan Lynch <nathanl@linux.ibm.com>,
Gautham R Shenoy <ego@linux.vnet.ibm.com>,
Peter Zijlstra <peterz@infradead.org>,
Daniel Henrique Barboza <danielhb413@gmail.com>,
Valentin Schneider <valentin.schneider@arm.com>,
hegdevasant@linux.vnet.ibm.com, qemu-ppc@nongnu.org,
Cedric Le Goater <clg@kaod.org>,
linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
Ingo Molnar <mingo@kernel.org>
Subject: Re: [PATCH 1/3] powerpc/smp: Reintroduce cpu_core_mask
Date: Fri, 16 Apr 2021 11:15:49 +0530 [thread overview]
Message-ID: <20210416054549.GF2633526@linux.vnet.ibm.com> (raw)
In-Reply-To: <YHkCvuNfZLQRxjU8@yekko.fritz.box>
* David Gibson <david@gibson.dropbear.id.au> [2021-04-16 13:21:34]:
Thanks for having a look at the patches.
> On Thu, Apr 15, 2021 at 05:39:32PM +0530, Srikar Dronamraju wrote:
> > Daniel reported that with Commit 4ca234a9cbd7 ("powerpc/smp: Stop
> > updating cpu_core_mask") QEMU was unable to set single NUMA node SMP
> > topologies such as:
> > -smp 8,maxcpus=8,cores=2,threads=2,sockets=2
> > i.e he expected 2 sockets in one NUMA node.
>
> Well, strictly speaking, you can still set that toplogy in qemu but a
> PAPR guest with that commit will show as having 1 socket in lscpu and
> similar things.
>
Right, I did mention the o/p of lscpu in QEMU with the said commit and
with the new patches in the cover letter. Somehow I goofed up the cc
list for the cover letter.
Reference for the cover letter:
https://lore.kernel.org/linuxppc-dev/20210415120934.232271-1-srikar@linux.vnet.ibm.com/t/#u
> Basically, this is because PAPR has no meaningful distinction between
> cores and sockets. So it's kind of a cosmetic problem, but it is a
> user-unexpected behaviour that it would be nice to avoid if it's not
> excessively difficult.
>
> > The above commit helped to reduce boot time on Large Systems for
> > example 4096 vCPU single socket QEMU instance. PAPR is silent on
> > having more than one socket within a NUMA node.
> >
> > cpu_core_mask and cpu_cpu_mask for any CPU would be same unless the
> > number of sockets is different from the number of NUMA nodes.
>
> Number of sockets being different from number of NUMA nodes is routine
> in qemu, and I don't think it's something we should enforce.
>
> > One option is to reintroduce cpu_core_mask but use a slightly
> > different method to arrive at the cpu_core_mask. Previously each CPU's
> > chip-id would be compared with all other CPU's chip-id to verify if
> > both the CPUs were related at the chip level. Now if a CPU 'A' is
> > found related / (unrelated) to another CPU 'B', all the thread
> > siblings of 'A' and thread siblings of 'B' are automatically marked as
> > related / (unrelated).
> >
> > Also if a platform doesn't support ibm,chip-id property, i.e its
> > cpu_to_chip_id returns -1, cpu_core_map holds a copy of
> > cpu_cpu_mask().
>
> Yeah, the other weirdness here is that ibm,chip-id isn't a PAPR
> property at all - it was added for powernv. We then added it to qemu
> for PAPR guests because that was the way at the time to get the guest
> to advertise the expected number of sockets. It therefore basically
> *only* exists on PAPR/qemu for that purpose, so if it's not serving it
> we need to come up with something else.
>
Do you have ideas on what that something could be like? So if that's
more beneficial then we could move over to that scheme. Also apart
from ibm,chip-id being not a PAPR property, do you have any other
concerns with it.
--
Thanks and Regards
Srikar Dronamraju
next prev parent reply other threads:[~2021-04-16 5:46 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-15 12:09 [PATCH 0/3] Reintroduce cpu_core_mask Srikar Dronamraju
2021-04-15 12:09 ` [PATCH 1/3] powerpc/smp: " Srikar Dronamraju
2021-04-15 17:11 ` Gautham R Shenoy
2021-04-15 17:36 ` Srikar Dronamraju
2021-04-16 3:21 ` David Gibson
2021-04-16 5:45 ` Srikar Dronamraju [this message]
2021-04-19 1:17 ` David Gibson
2021-04-15 12:09 ` [PATCH 2/3] Revert "powerpc/topology: Update topology_core_cpumask" Srikar Dronamraju
2021-04-15 12:09 ` [PATCH 3/3] powerpc/smp: Cache CPU to chip lookup Srikar Dronamraju
2021-04-15 17:19 ` Gautham R Shenoy
2021-04-15 17:51 ` Srikar Dronamraju
2021-04-16 15:57 ` Gautham R Shenoy
2021-04-16 16:57 ` Srikar Dronamraju
2021-04-19 1:19 ` David Gibson
2021-04-15 12:17 ` [PATCH 0/3] Reintroduce cpu_core_mask Daniel Henrique Barboza
2021-04-19 4:00 ` Michael Ellerman
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