All of lore.kernel.org
 help / color / mirror / Atom feed
From: Vitaly Kuznetsov <vkuznets@redhat.com>
To: Michael Kelley <mikelley@microsoft.com>
Cc: "catalin.marinas\@arm.com" <catalin.marinas@arm.com>,
	"mark.rutland\@arm.com" <mark.rutland@arm.com>,
	"will.deacon\@arm.com" <will.deacon@arm.com>,
	"marc.zyngier\@arm.com" <marc.zyngier@arm.com>,
	"linux-arm-kernel\@lists.infradead.org" 
	<linux-arm-kernel@lists.infradead.org>,
	"gregkh\@linuxfoundation.org" <gregkh@linuxfoundation.org>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-hyperv\@vger.kernel.org" <linux-hyperv@vger.kernel.org>,
	"olaf\@aepfle.de" <olaf@aepfle.de>,
	"apw\@canonical.com" <apw@canonical.com>,
	"jasowang\@redhat.com" <jasowang@redhat.com>,
	"marcelo.cerri\@canonical.com" <marcelo.cerri@canonical.com>,
	Sunil Muthuswamy <sunilmut@microsoft.com>,
	KY Srinivasan <kys@microsoft.com>
Subject: RE: [PATCH v3 2/2] Drivers: hv: Move Hyper-V clocksource code to new clocksource driver
Date: Thu, 30 May 2019 15:51:05 +0200	[thread overview]
Message-ID: <87imtskq46.fsf@vitty.brq.redhat.com> (raw)
In-Reply-To: <BYAPR21MB122115920E78B7897FDC7BE9D7180@BYAPR21MB1221.namprd21.prod.outlook.com>

Michael Kelley <mikelley@microsoft.com> writes:

> From: Vitaly Kuznetsov <vkuznets@redhat.com> Sent: Thursday, May 30, 2019 2:48 AM
>> > +		/*
>> > +		 * sched_clock_register is needed on ARM64 but
>> > +		 * is a no-op on x86
>> > +		 */
>> > +		sched_clock_register(read_hv_sched_clock_msr,
>> > +						64, HV_CLOCK_HZ);
>> 
>> I'm not sure about ARM, but MSR-based clocksource would be a really bad
>> choice for sched clock on x86, this will slow things down
>> significantly. Luckily, as you're validly stating above,
>> sched_clock_register() is a no-op on x86 as we don't define
>> CONFIG_GENERIC_SCHED_CLOCK.
>> 
>> Can we actually *not* do sched_clock_register() in case
>> TSC page is unavailable (and revert to counting jiffies or whatever)?
>> 
>
> We can't skip the sched_clock_register() on ARM64 because it
> does define CONFIG_GENERIC_SCHED_CLOCK.  However, Hyper-V
> should always provide REFERENCE_TSC_AVAILALBE on ARM64,
> so we should never end up in the MSR-based code on ARM64.
> Arguably that means the call to sched_clock_register() could be
> removed since it's a no-op on x86.  But I'd like to keep it for symmetry
> and in case there's a testing/debugging situation on ARM64 where
> we want to clear REFERENCE_TSC_AVAILABLE and go down the
> MSR-based code path.

Ok, so it is just a fall-back and not going to be actively used. Thanks!

-- 
Vitaly

WARNING: multiple messages have this Message-ID (diff)
From: Vitaly Kuznetsov <vkuznets@redhat.com>
To: Michael Kelley <mikelley@microsoft.com>
Cc: "mark.rutland@arm.com" <mark.rutland@arm.com>,
	"linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>,
	"marc.zyngier@arm.com" <marc.zyngier@arm.com>,
	"catalin.marinas@arm.com" <catalin.marinas@arm.com>,
	"jasowang@redhat.com" <jasowang@redhat.com>,
	"will.deacon@arm.com" <will.deacon@arm.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"marcelo.cerri@canonical.com" <marcelo.cerri@canonical.com>,
	"olaf@aepfle.de" <olaf@aepfle.de>,
	"gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>,
	"apw@canonical.com" <apw@canonical.com>,
	Sunil Muthuswamy <sunilmut@microsoft.com>,
	KY Srinivasan <kys@microsoft.com>,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>
Subject: RE: [PATCH v3 2/2] Drivers: hv: Move Hyper-V clocksource code to new clocksource driver
Date: Thu, 30 May 2019 15:51:05 +0200	[thread overview]
Message-ID: <87imtskq46.fsf@vitty.brq.redhat.com> (raw)
In-Reply-To: <BYAPR21MB122115920E78B7897FDC7BE9D7180@BYAPR21MB1221.namprd21.prod.outlook.com>

Michael Kelley <mikelley@microsoft.com> writes:

> From: Vitaly Kuznetsov <vkuznets@redhat.com> Sent: Thursday, May 30, 2019 2:48 AM
>> > +		/*
>> > +		 * sched_clock_register is needed on ARM64 but
>> > +		 * is a no-op on x86
>> > +		 */
>> > +		sched_clock_register(read_hv_sched_clock_msr,
>> > +						64, HV_CLOCK_HZ);
>> 
>> I'm not sure about ARM, but MSR-based clocksource would be a really bad
>> choice for sched clock on x86, this will slow things down
>> significantly. Luckily, as you're validly stating above,
>> sched_clock_register() is a no-op on x86 as we don't define
>> CONFIG_GENERIC_SCHED_CLOCK.
>> 
>> Can we actually *not* do sched_clock_register() in case
>> TSC page is unavailable (and revert to counting jiffies or whatever)?
>> 
>
> We can't skip the sched_clock_register() on ARM64 because it
> does define CONFIG_GENERIC_SCHED_CLOCK.  However, Hyper-V
> should always provide REFERENCE_TSC_AVAILALBE on ARM64,
> so we should never end up in the MSR-based code on ARM64.
> Arguably that means the call to sched_clock_register() could be
> removed since it's a no-op on x86.  But I'd like to keep it for symmetry
> and in case there's a testing/debugging situation on ARM64 where
> we want to clear REFERENCE_TSC_AVAILABLE and go down the
> MSR-based code path.

Ok, so it is just a fall-back and not going to be actively used. Thanks!

-- 
Vitaly

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

  reply	other threads:[~2019-05-30 13:51 UTC|newest]

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-27 14:59 [PATCH v3 0/2] Drivers: hv: Move Hyper-V clock/timer code to separate clocksource driver Michael Kelley
2019-05-27 14:59 ` Michael Kelley
2019-05-27 14:59 ` [PATCH v3 1/2] Drivers: hv: Create Hyper-V clocksource driver from existing clockevents code Michael Kelley
2019-05-27 14:59   ` Michael Kelley
2019-05-30  9:37   ` Vitaly Kuznetsov
2019-05-30  9:37     ` Vitaly Kuznetsov
2019-05-27 14:59 ` [PATCH v3 2/2] Drivers: hv: Move Hyper-V clocksource code to new clocksource driver Michael Kelley
2019-05-27 14:59   ` Michael Kelley
2019-05-30  9:48   ` Vitaly Kuznetsov
2019-05-30  9:48     ` Vitaly Kuznetsov
2019-05-30 12:39     ` Michael Kelley
2019-05-30 12:39       ` Michael Kelley
2019-05-30 13:51       ` Vitaly Kuznetsov [this message]
2019-05-30 13:51         ` Vitaly Kuznetsov
2019-06-06 16:37 ` [PATCH v3 0/2] Drivers: hv: Move Hyper-V clock/timer code to separate " Sasha Levin
2019-06-06 16:37   ` Sasha Levin

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=87imtskq46.fsf@vitty.brq.redhat.com \
    --to=vkuznets@redhat.com \
    --cc=apw@canonical.com \
    --cc=catalin.marinas@arm.com \
    --cc=gregkh@linuxfoundation.org \
    --cc=jasowang@redhat.com \
    --cc=kys@microsoft.com \
    --cc=linux-arm-kernel@lists.infradead.org \
    --cc=linux-hyperv@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=marc.zyngier@arm.com \
    --cc=marcelo.cerri@canonical.com \
    --cc=mark.rutland@arm.com \
    --cc=mikelley@microsoft.com \
    --cc=olaf@aepfle.de \
    --cc=sunilmut@microsoft.com \
    --cc=will.deacon@arm.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body. 
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.