linux-kernel - Re: [RFC/PATCH 1/3] PM: QoS: Introduce a system-wakeup QoS limit

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <CAJZ5v0j=9RXHrcVEBp0yy1Ae4_kC1y-WFQyBf89r3NtoL-tYQw@mail.gmail.com>
Date: Mon, 11 Aug 2025 21:16:34 +0200
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Kevin Hilman <khilman@...libre.com>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>, Ulf Hansson <ulf.hansson@...aro.org>, linux-pm@...r.kernel.org, 
	Pavel Machek <pavel@...nel.org>, Len Brown <len.brown@...el.com>, 
	Daniel Lezcano <daniel.lezcano@...aro.org>, Saravana Kannan <saravanak@...gle.com>, 
	Maulik Shah <quic_mkshah@...cinc.com>, Prasad Sodagudi <psodagud@...cinc.com>, 
	linux-kernel@...r.kernel.org
Subject: Re: [RFC/PATCH 1/3] PM: QoS: Introduce a system-wakeup QoS limit

On Mon, Aug 11, 2025 at 7:16 PM Kevin Hilman <khilman@...libre.com> wrote:
>
> "Rafael J. Wysocki" <rafael@...nel.org> writes:
>
> > On Wed, Jul 16, 2025 at 2:33 PM Ulf Hansson <ulf.hansson@...aro.org> wrote:
> >>
> >> Some platforms and devices supports multiple low-power-states than can be
> >> used for system-wide suspend. Today these states are selected on per
> >> subsystem basis and in most cases it's the deepest possible state that
> >> becomes selected.
> >>
> >> For some use-cases this is a problem as it isn't suitable or even breaks
> >> the system-wakeup latency constraint, when we decide to enter these deeper
> >> states during system-wide suspend.
> >>
> >> Therefore, let's introduce an interface for user-space, allowing us to
> >> specify the system-wakeup QoS limit. Subsequent changes will start taking
> >> into account the QoS limit.
> >
> > Well, this is not really a system-wakeup limit, but a CPU idle state
> > latency limit for states entered in the last step of suspend-to-idle.
> >
> > It looks like the problem is that the existing CPU latency QoS is not
> > taken into account by suspend-to-idle, so instead of adding an
> > entirely new interface to overcome this, would it make sense to add an
> > ioctl() to the existing one that would allow the user of it to
> > indicate that the given request should also be respected by
> > suspend-to-idle?
> >
> > There are two basic reasons why I think so:
> > (1) The requests that you want to be respected by suspend-to-idle
> > should also be respected by the regular "runtime" idle, or at least I
> > don't see a reason why it wouldn't be the case.
> > (2) The new interface introduced by this patch basically duplicates
> > the existing one.
>
> I also think that just using the existing /dev/cpu_dma_latency is the
> right approach here, and simply teaching s2idle to respect this value.
>
> I'm curious about the need for a new ioctl() though.  Under what
> conditions do you want normal/runtime CPUidle to respect this value and
> s2idle to not respect this value?

In a typical PC environment s2idle is a replacement for ACPI S3 which
does not take any QoS constraints into account, so users may want to
set QoS limits for run-time and then suspend with the expectation that
QoS will not affect it.