From: Andrew Cooper <andrew.cooper3@citrix.com>
To: "Roger Pau Monné" <roger.pau@citrix.com>,
"Igor Druzhinin" <igor.druzhinin@citrix.com>
Cc: xen-devel@lists.xenproject.org, wl@xen.org, jbeulich@suse.com
Subject: Re: [Xen-devel] [PATCH 1/2] x86/time: drop vtsc_{kern, user}count debug counters
Date: Mon, 16 Dec 2019 14:24:28 +0000 [thread overview]
Message-ID: <41900202-789c-0f42-787f-a9317f8687ee@citrix.com> (raw)
In-Reply-To: <20191216094744.GL11756@Air-de-Roger>
On 16/12/2019 09:47, Roger Pau Monné wrote:
> On Fri, Dec 13, 2019 at 10:48:01PM +0000, Igor Druzhinin wrote:
>> They either need to be transformed to atomics to work correctly
>> (currently they left unprotected for HVM domains) or dropped entirely
> ^ are used
>> as taking a per-domain spinlock is too expensive for high-vCPU count
>> domains even for debug build given this lock is taken too often.
>>
>> Choose the latter as they are not extremely important anyway.
>>
>> Signed-off-by: Igor Druzhinin <igor.druzhinin@citrix.com>
> I don't find those counters specially useful TBH, but I'm not sure whether
> others find them useful. The change LGTM, so:
>
> Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Jan and I already considered dropping them once (because of the HVM
observation), but didn't find any harm with keeping them, seeing as they
were diagnostic-only.
I suspect they were put in for PVRDTSCP which has since been dropped.
We now have a clear case where dropping them is of use to Xen.
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
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next prev parent reply other threads:[~2019-12-16 14:25 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-12-13 22:48 [Xen-devel] [PATCH 0/2] vTSC performance improvements Igor Druzhinin
2019-12-13 22:48 ` [Xen-devel] [PATCH 1/2] x86/time: drop vtsc_{kern, user}count debug counters Igor Druzhinin
2019-12-16 9:47 ` Roger Pau Monné
2019-12-16 14:24 ` Andrew Cooper [this message]
2019-12-13 22:48 ` [Xen-devel] [PATCH 2/2] x86/time: update vtsc_last with cmpxchg and drop vtsc_lock Igor Druzhinin
2019-12-16 10:00 ` Roger Pau Monné
2019-12-16 11:21 ` Jan Beulich
2019-12-16 12:30 ` Roger Pau Monné
2019-12-16 12:45 ` Jan Beulich
2019-12-16 12:55 ` Roger Pau Monné
2019-12-16 12:53 ` Igor Druzhinin
2019-12-16 12:57 ` Roger Pau Monné
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