Re: [PATCH v4 12/12] xen/spinlock: support higher number of cpus

From: Jan Beulich <jbeulich@suse.com>
To: "Jürgen Groß" <jgross@suse.com>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>,
	George Dunlap <george.dunlap@citrix.com>,
	Julien Grall <julien@xen.org>,
	Stefano Stabellini <sstabellini@kernel.org>, Wei Liu <wl@xen.org>,
	xen-devel@lists.xenproject.org
Subject: Re: [PATCH v4 12/12] xen/spinlock: support higher number of cpus
Date: Thu, 29 Feb 2024 17:31:36 +0100	[thread overview]
Message-ID: <f2d107a1-0bcf-4d71-89e8-819ee38e0651@suse.com> (raw)
In-Reply-To: <f6658df0-13f2-4129-afcd-0f122c87132b@suse.com>

On 29.02.2024 17:29, Jürgen Groß wrote:
> On 29.02.24 16:46, Jan Beulich wrote:
>> On 12.12.2023 10:47, Juergen Gross wrote:
>>> Allow 16 bits per cpu number, which is the limit imposed by
>>> spinlock_tickets_t.
>>>
>>> This will allow up to 65535 cpus, while increasing only the size of
>>> recursive spinlocks in debug builds from 8 to 12 bytes.
>>
>> I think we want to be more conservative here, for the case of there
>> being bugs: The CPU holding a lock may wrongly try to acquire it a
>> 2nd time. That's the 65536th ticket then, wrapping the value.
> 
> Is this really a problem? There will be no other cpu left seeing the lock
> as "free" in this case, as all others will be waiting for the head to reach
> their private tail value.

But isn't said CPU then going to make progress, rather than indefinitely
spinning on the lock?

>> Therefore my suggestion would be to only (mention) go(ing) up to 32k.
>>
>>> Signed-off-by: Juergen Gross <jgross@suse.com>
>>> ---
>>>   xen/common/spinlock.c      |  1 +
>>>   xen/include/xen/spinlock.h | 18 +++++++++---------
>>>   2 files changed, 10 insertions(+), 9 deletions(-)
>>
>> Shouldn't this also bump the upper bound of the NR_CPUS range then
>> in xen/arch/Kconfig?
> 
> Fine with me, I can add another patch to the series doing that.

Why not do it right here? The upper bound there is like it is only
because of the restriction that's lifted here.

Jan