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Subject: Re: [External] : Re: [PATCH v14 4/6] locking/qspinlock: Introduce
 starvation avoidance into CNA
From:   Alex Kogan <alex.kogan@oracle.com>
In-Reply-To: <87mtu2vhzz.fsf@linux.intel.com>
Date:   Tue, 13 Apr 2021 17:01:37 -0400
Cc:     linux@armlinux.org.uk, Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>, will.deacon@arm.com,
        arnd@arndb.de, longman@redhat.com, linux-arch@vger.kernel.org,
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        dave.dice@oracle.com
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References: <20210401153156.1165900-1-alex.kogan@oracle.com>
 <20210401153156.1165900-5-alex.kogan@oracle.com>
 <87mtu2vhzz.fsf@linux.intel.com>
To:     Andi Kleen <ak@linux.intel.com>
Precedence: bulk

Hi, Andi.

Thanks for your comments!

> On Apr 13, 2021, at 2:03 AM, Andi Kleen <ak@linux.intel.com> wrote:
>=20
> Alex Kogan <alex.kogan@oracle.com> writes:
>>=20
>> +	numa_spinlock_threshold=3D	[NUMA, PV_OPS]
>> +			Set the time threshold in milliseconds for the
>> +			number of intra-node lock hand-offs before the
>> +			NUMA-aware spinlock is forced to be passed to
>> +			a thread on another NUMA node.	Valid values
>> +			are in the [1..100] range. Smaller values result
>> +			in a more fair, but less performant spinlock,
>> +			and vice versa. The default value is 10.
>=20
> ms granularity seems very coarse grained for this. Surely
> at some point of spinning you can afford a ktime_get? But ok.
We are reading time when we are at the head of the (main) queue, but
don=E2=80=99t have the lock yet. Not sure about the latency of =
ktime_get(), but
anything reasonably fast but not necessarily precise should work.

> Could you turn that into a moduleparm which can be changed at runtime?
> Would be strange to have to reboot just to play with this parameter
Yes, good suggestion, thanks.

> This would also make the code a lot shorter I guess.
So you don=E2=80=99t think we need the command-line parameter, just the =
module_param?

Regards,
=E2=80=94 Alex