From: Marcelo Tosatti <mtosatti@redhat.com>
To: Alex Belits <abelits@marvell.com>
Cc: Christoph Lameter <cl@linux.com>,
"tglx@linutronix.de" <tglx@linutronix.de>,
"pauld@redhat.com" <pauld@redhat.com>,
"linux-mm@kvack.org" <linux-mm@kvack.org>,
"frederic@kernel.org" <frederic@kernel.org>,
"willy@infradead.org" <willy@infradead.org>,
"peterz@infradead.org" <peterz@infradead.org>,
"akpm@linux-foundation.org" <akpm@linux-foundation.org>,
Juri Lelli <juri.lelli@redhat.com>,
Daniel Bristot de Oliveira <bristot@redhat.com>,
Nitesh Narayan Lal <nitesh@redhat.com>
Subject: Re: [RFC] tentative prctl task isolation interface
Date: Fri, 22 Jan 2021 10:05:11 -0300 [thread overview]
Message-ID: <20210122130511.GA58675@fuller.cnet> (raw)
In-Reply-To: <20210121162059.GA18719@fuller.cnet>
On Thu, Jan 21, 2021 at 01:20:59PM -0300, Marcelo Tosatti wrote:
>
> Adding Nitesh to CC.
>
> On Thu, Jan 21, 2021 at 12:51:41PM -0300, Marcelo Tosatti wrote:
> > Hi Alex,
> >
> > On Fri, Jan 15, 2021 at 10:35:14AM -0800, Alex Belits wrote:
> > > On 1/15/21 05:24, Christoph Lameter wrote:
> > >
> > > > ----------------------------------------------------------------------
> > > > On Thu, 14 Jan 2021, Marcelo Tosatti wrote:
> > > >
> > > > > > How does one do a oneshot flush of OS activities?
> > > > >
> > > > > ret = prctl(PR_TASK_ISOLATION_REQUEST, ISOL_F_QUIESCE, 0, 0, 0);
> > > > > if (ret == -1) {
> > > > > perror("prctl PR_TASK_ISOLATION_REQUEST");
> > > > > exit(0);
> > > > > }
> > > > >
> > > > > >
> > > > > > I.e. I have a polling loop over numerous shared and I/o devices in user
> > > > > > space and I want to make sure that the system is quite before I enter the
> > > > > > loop.
> > > > >
> > > > > You could configure things in two ways: with syscalls allowed or not.
> > > >
> > > > Well syscalls that do not cause deferred processing like getting the time
> > > > or determining the current cpu should be ok to use.
> > >
> > > Some of those syscalls go through vdso, and don't enter the kernel --
> > > nothing specific is necessary to allow them, and it would be pointless and
> > > difficult to prevent them.
> > >
> > > For syscalls that enter the kernel, it's often difficult to predict, if they
> > > will or won't cause deferred processing, so I am afraid, it won't be
> > > possible to provide a "safe" class of syscalls for this purpose and not end
> > > up with something minimal like reading /sys and /proc. Right now isolation
> > > only "allows" syscalls that exit isolation.
> >
> > Christoph wrote:
> >
> > "> Features that I think may be needed:
> > >
> > > F_ISOL_QUIESCE -> quiet down now but allow all OS activities. OS
> > > activites reset flag
> > >
> > > F_ISOL_BAREMETAL_HARD -> No OS interruptions. Fault on syscalls that
> > > require such actions in the future.
> > >
> > > F_ISOL_BAREMETAL_WARN -> Similar. Create a warning in the syslog when OS
> > > services require delayed processing etc
> > > but continue while resetting the flag.
> > "
> >
> > It seems the only difference between HARD and WARN (lets call it SOFT)
> > would be whether a notification is sent to userspace.
> >
> > The definition
> >
> > "F_ISOL_BAREMETAL_HARD -> No OS interruptions. Fault on syscalls that
> > require such actions in the future."
> >
> > fails in the static_key_enable case: Alex's idea is to queue the i-cache
> > flush if the remote task/cpu is in isolated mode (and perform the flush
> > when entering the kernel).
> >
> > So even if userspace uses syscalls that do not require delayed
> > processing, there are events which are out of control of the
> > application and might require it.
> >
> > So lets assume the application performs a number of syscalls on a
> > given time critical codepath.
> >
> > Either the system is configured so that
> > the number/frequency of static_key_enable's is limited, or the cost of
> > i-cache flushes must be accounted on that critical codepath.
> >
> > Anyway, trying to improve Christoph's definition:
> >
> > F_ISOL_QUIESCE -> flush any pending operations that might cause
> > the CPU to be interrupted (ex: free's
> > per-CPU queues, sync MM statistics
> > counters, etc).
> >
> > F_ISOL_ISOLATE -> inform the kernel that userspace is
> > entering isolated mode (see description
> > below on "ISOLATION MODES").
> >
> > F_ISOL_UNISOLATE -> inform the kernel that userspace is
> > leaving isolated mode.
> >
> > F_ISOL_NOTIFY -> notification mode of isolation breakage
> > modes.
> >
> >
> > Isolation modes:
> > ---------------
> >
> > There are two main types of isolation modes:
> >
> > - SOFT mode: does not prevent activities which might generate interruptions
> > (such as CPU hotplug).
> >
> > - HARD mode: prevents all blockable activities that might generate interruptions.
> > Administrators can override this via /sys.
> >
> > Notifications:
> > -------------
> >
> > Notification mode of isolation breakage can be configured as follows:
> >
> > - None (default): No notification is performed by the kernel on isolation
> > breakage.
> >
> > - Syslog: Isolation breakage is reported to syslog.
> >
> > (new modes can be added, for example signals).
> >
> > A new feature can be added to disallow syscalls (by default syscalls
> > are enabled, with reporting of pending activities that might cause
> > an interruption in a VDSO).
After discussion with Juri and Daniel, it became clearer that supporting
unmodified applications would be quite useful:
- enter isolation mode
- run unmodified application
- leave isolation mode
This could work via an additional mode which goes through the quiesce
operation at every syscall return. Since this includes freeing per-CPU
pagevecs (therefore allocating per-CPU pagevecs at the next syscall),
it might considerably slowdown system startup (and cause MM related
spinlocks contention).
Better ideas are appreciated.
next prev parent reply other threads:[~2021-01-22 13:25 UTC|newest]
Thread overview: 42+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-17 16:28 [PATCH] mm: introduce sysctl file to flush per-cpu vmstat statistics Marcelo Tosatti
2020-11-17 18:03 ` Matthew Wilcox
2020-11-17 19:06 ` Christopher Lameter
2020-11-17 19:09 ` Matthew Wilcox
2020-11-20 18:04 ` Christopher Lameter
2020-11-17 20:23 ` Marcelo Tosatti
2020-11-20 18:02 ` Marcelo Tosatti
2020-11-20 18:20 ` Christopher Lameter
2020-11-23 18:02 ` Marcelo Tosatti
2020-11-24 17:12 ` Vlastimil Babka
2020-11-24 19:52 ` Marcelo Tosatti
2020-11-27 15:48 ` Marcelo Tosatti
2020-11-28 3:49 ` [EXT] " Alex Belits
2020-11-30 18:18 ` Marcelo Tosatti
2020-11-30 18:29 ` Marcelo Tosatti
2020-12-03 22:47 ` Alex Belits
2020-12-03 22:21 ` Alex Belits
2020-11-30 9:31 ` Christoph Lameter
2020-12-02 12:43 ` Marcelo Tosatti
2020-12-02 15:57 ` Thomas Gleixner
2020-12-02 17:43 ` Christoph Lameter
2020-12-03 3:17 ` Thomas Gleixner
2020-12-07 8:08 ` Christoph Lameter
2020-12-07 16:09 ` Thomas Gleixner
2020-12-07 19:01 ` Thomas Gleixner
2020-12-02 18:38 ` Marcelo Tosatti
2020-12-04 0:20 ` Frederic Weisbecker
2020-12-04 13:31 ` Marcelo Tosatti
2020-12-04 1:43 ` [EXT] " Alex Belits
2021-01-13 12:15 ` [RFC] tentative prctl task isolation interface Marcelo Tosatti
2021-01-14 9:22 ` Christoph Lameter
2021-01-14 19:34 ` Marcelo Tosatti
2021-01-15 13:24 ` Christoph Lameter
2021-01-15 18:35 ` Alex Belits
2021-01-21 15:51 ` Marcelo Tosatti
2021-01-21 16:20 ` Marcelo Tosatti
2021-01-22 13:05 ` Marcelo Tosatti [this message]
2021-02-01 10:48 ` Christoph Lameter
2021-02-01 12:47 ` Alex Belits
2021-02-01 18:20 ` Marcelo Tosatti
2021-01-18 15:18 ` Marcelo Tosatti
2020-11-24 5:02 ` [mm] e655d17ffa: BUG:using_smp_processor_id()in_preemptible kernel test robot
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