linux-kernel - Re: [patch v7 02/10] add prctl task isolation prctl docs and samples

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Date:   Tue, 23 Nov 2021 15:37:26 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Marcelo Tosatti <mtosatti@...hat.com>
Cc:     linux-kernel@...r.kernel.org, Nitesh Lal <nilal@...hat.com>,
        Nicolas Saenz Julienne <nsaenzju@...hat.com>,
        Christoph Lameter <cl@...ux.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Alex Belits <abelits@...its.com>, Peter Xu <peterx@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>
Subject: Re: [patch v7 02/10] add prctl task isolation prctl docs and samples

On Fri, Nov 12, 2021 at 09:35:33AM -0300, Marcelo Tosatti wrote:
> +**PR_ISOL_CFG_GET**:
> +
> +        Retrieve task isolation configuration.
> +        The general format is::
> +
> +                prctl(PR_ISOL_CFG_GET, what, arg3, arg4, arg5);
> +
> +        The 'what' argument specifies what to configure. Possible values are:
> +
> +        - ``I_CFG_FEAT``:
> +
> +                Return configuration of task isolation features. The 'arg3' argument specifies
> +                whether to return configured features (if zero), or individual
> +                feature configuration (if not zero), as follows.
> +
> +                - ``0``:
> +
> +                        Return the bitmask of configured features, in the location
> +                        pointed  to  by  ``(int *)arg4``. The buffer should allow space
> +                        for 8 bytes.
> +
> +                - ``ISOL_F_QUIESCE``:
> +
> +                        If arg4 is QUIESCE_CONTROL, return the control structure for
> +                        quiescing of background kernel activities, in the location
> +                        pointed to by ``(int *)arg5``::
> +
> +                         struct task_isol_quiesce_control {
> +                                __u64 flags;
> +                                __u64 quiesce_mask;
> +                                __u64 quiesce_oneshot_mask;
> +                                __u64 pad[5];
> +                         };
> +
> +                        See PR_ISOL_CFG_GET description for meaning of
> fields.

PR_ISOL_CFG_SET ?

[...]
> +
> +                        *quiesce_oneshot_mask*: A bitmask indicating which kernel
> +                        activities should behave in oneshot mode, that is, quiescing
> +                        will happen on return from prctl(PR_ISOL_ACTIVATE_SET), but not
> +                        on return of subsequent system calls. The corresponding bit(s)
> +                        must also be set at quiesce_mask.

Don't forget to mention interrupts and exceptions.

> +
> +                        *pad*: Additional space for future enhancements.
> +
> +                        For quiesce_mask (and quiesce_oneshot_mask), possible bit sets are:
> +
> +                        - ``ISOL_F_QUIESCE_VMSTATS``
> +
> +                        VM statistics are maintained in per-CPU counters to
> +                        improve performance. When a CPU modifies a VM statistic,
> +                        this modification is kept in the per-CPU counter.
> +                        Certain activities require a global count, which
> +                        involves requesting each CPU to flush its local counters
> +                        to the global VM counters.
> +
> +                        This flush is implemented via a workqueue item, which
> +                        might schedule a workqueue on isolated CPUs.
> +
> +                        To avoid this interruption, task isolation can be
> +                        configured to, upon return from system calls, synchronize
> +                        the per-CPU counters to global counters, thus avoiding
> +                        the interruption.
> +
> +        - ``I_CFG_INHERIT``:
> +                Set inheritance configuration when a new task
> +                is created via fork and clone.
> +
> +                The ``(int *)arg4`` argument is a pointer to::
> +
> +                        struct task_isol_inherit_control {
> +                                __u8    inherit_mask;
> +                                __u8    pad[7];
> +                        };
> +
> +                inherit_mask is a bitmask that specifies which part
> +                of task isolation should be inherited:
> +
> +                - Bit ISOL_INHERIT_CONF: Inherit task isolation configuration.
> +                  This is the state written via prctl(PR_ISOL_CFG_SET, ...).
> +
> +                - Bit ISOL_INHERIT_ACTIVE: Inherit task isolation activation
> +                  (requires ISOL_INHERIT_CONF to be set). The new task
> +                  should behave, after fork/clone, in the same manner
> +                  as the parent task after it executed:
> +
> +                        prctl(PR_ISOL_ACTIVATE_SET, &mask, ...);

I'm confused, what is the purpose of ISOL_INHERIT_CONF?

Thanks.