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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: <20210413212203.GT3762101@tassilo.jf.intel.com>
Date:   Tue, 13 Apr 2021 22:30:41 -0400
Cc:     linux@armlinux.org.uk, Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Will Deacon <will.deacon@arm.com>, arnd@arndb.de,
        longman@redhat.com, linux-arch@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        tglx@linutronix.de, bp@alien8.de, hpa@zytor.com, x86@kernel.org,
        guohanjun@huawei.com, jglauber@marvell.com,
        steven.sistare@oracle.com, daniel.m.jordan@oracle.com,
        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>
 <CA1141EF-76A8-47A9-97B9-3CB2FC246B1A@oracle.com>
 <20210413212203.GT3762101@tassilo.jf.intel.com>
To:     Andi Kleen <ak@linux.intel.com>
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> On Apr 13, 2021, at 5:22 PM, Andi Kleen <ak@linux.intel.com> wrote:
>=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.
>=20
> Actually cpu_clock / sched_clock (see my other email). These should
> be fast without corner cases and also monotonic.
I see, thanks.

>=20
>>=20
>>> 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.
>>=20
>>> 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?
>=20
> module_params can be changed at the command line too, so yes.
Got it, thanks again.

=E2=80=94 Alex