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    SubjectRe: [patch 1/4] add basic task isolation prctl interface
    On Wed, Jul 28, 2021 at 01:55:33PM +0200, nsaenzju@redhat.com wrote:
    > Hi Marcelo,
    >
    > On Wed, 2021-07-28 at 06:37 -0300, Marcelo Tosatti wrote:
    > > On Wed, Jul 28, 2021 at 01:45:39AM +0200, Frederic Weisbecker wrote:
    > > > On Tue, Jul 27, 2021 at 11:52:09AM -0300, Marcelo Tosatti wrote:
    > > > > The meaning of isolated is specified as follows:
    > > > >
    > > > > Isolation features
    > > > > ==================
    > > > >
    > > > > - prctl(PR_ISOL_GET, ISOL_SUP_FEATURES, 0, 0, 0) returns the supported
    > > > > features as a return value.
    > > > >
    > > > > - prctl(PR_ISOL_SET, ISOL_FEATURES, bitmask, 0, 0) enables the features in
    > > > > the bitmask.
    > > > >
    > > > > - prctl(PR_ISOL_GET, ISOL_FEATURES, 0, 0, 0) returns the currently
    > > > > enabled features.
    > > >
    > > > So what are the ISOL_FEATURES here? A mode that we enter such as flush
    > > > vmstat _everytime_ we resume to userpace after (and including) this prctl() ?
    > >
    > > ISOL_FEATURES is just the "command" type (which you can get and set).
    > >
    > > The bitmask would include ISOL_F_QUIESCE_ON_URET, so:
    > >
    > > - bitmask = ISOL_F_QUIESCE_ON_URET;
    > > - prctl(PR_ISOL_SET, ISOL_FEATURES, bitmask, 0, 0) enables the features in
    > > the bitmask.
    > >
    > > - quiesce_bitmap = prctl(PR_ISOL_GET, PR_ISOL_SUP_QUIESCE_CFG, 0, 0, 0)
    > > (1)
    > >
    > > (returns the supported actions to be quiesced).
    > >
    > > - prctl(PR_ISOL_SET, PR_ISOL_QUIESCE_CFG, quiesce_bitmask, 0, 0) _sets_
    > > the actions to be quiesced (2)
    > >
    > > If an application does not modify "quiesce_bitmask" between
    > > points (1) and (2) above, it will enable quiescing of all
    > > "features" the kernel supports.
    >
    > I think this pattern of enabling all by default might be prone to subtly
    > breaking things.

    The reasoning behind this pattern is that many latency sensitive applications
    (as far as i am aware) prefer "as few interruptions as possible, no
    interruptions is preferred".

    In that case, the pattern makes sense.

    > For example, let's say we introduce ISOL_F_QUIESCE_DEFER_TLB_FLUSH, this will
    > defer relatively short IPIs on isolated CPUs in exchange for a longer flush
    > whenever we enter the kernel (syscall, IRQs, NMI, etc...).

    Why the flush has to be longer when you enter the kernel?

    ISOL_F_QUIESCE_DEFER_TLB_FLUSH might collapse multiple IPIs
    into a single IPI, so the behaviour might be beneficial
    for "standard" types of application as well.

    > A latency sensitive
    > application might be OK with the former but not with the latter.

    Two alternatives:

    1) The pattern above, where particular subsystems that might interrupt
    the kernel are enabled automatically if the kernel supports it.

    Pros:
    Applications which implement this only need to be changed once,
    and can benefit from new kernel features.

    Applications can disable particular features if they turn
    out to be problematic.

    Cons:
    New features might break applications.

    2) Force applications to enable each new feature individually.

    Pros: Won't cause regressions, kernel behaviour is explicitly
    controlled by userspace.

    Cons: Apps won't benefit from new features automatically.

    ---

    It seems to me 1) is preferred. Can also add a sysfs control to
    have a "default_isolation_feature" flag, which can be changed
    by a sysadmin in case a new feature is undesired.

    Thoughts?

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