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    SubjectRe: [RFC][PATCH 01/13] sched/deadline: Impose global limits on sched_attr::sched_period
    Hi Peter,

    This is indeed an important missing piece.

    I think it would be worth having some simple checks, e.g.:
    - period_min <= period_max;
    - period_min >= (1ULL << DL_SCALE).

    To be even more picky, I'm wondering if it would be a good practice to
    fail the write operation if there are already dl-tasks in the system
    that violate the new constraint. I'm afraid there is no cheap way of
    achieving such check, so, I think we can skip this last tricky thing for
    now.

    Thanks,
    Alessio

    On Fri, Jul 26, 2019 at 04:54:10PM +0200, Peter Zijlstra wrote:
    >
    > Cc: Daniel Bristot de Oliveira <bristot@redhat.com>
    > Cc: Luca Abeni <luca.abeni@santannapisa.it>
    > Cc: Juri Lelli <juri.lelli@redhat.com>
    > Cc: Dmitry Vyukov <dvyukov@google.com>
    > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    > ---
    > include/linux/sched/sysctl.h | 3 +++
    > kernel/sched/deadline.c | 23 +++++++++++++++++++++--
    > kernel/sysctl.c | 14 ++++++++++++++
    > 3 files changed, 38 insertions(+), 2 deletions(-)
    >
    > --- a/include/linux/sched/sysctl.h
    > +++ b/include/linux/sched/sysctl.h
    > @@ -56,6 +56,9 @@ int sched_proc_update_handler(struct ctl
    > extern unsigned int sysctl_sched_rt_period;
    > extern int sysctl_sched_rt_runtime;
    >
    > +extern unsigned int sysctl_sched_dl_period_max;
    > +extern unsigned int sysctl_sched_dl_period_min;
    > +
    > #ifdef CONFIG_UCLAMP_TASK
    > extern unsigned int sysctl_sched_uclamp_util_min;
    > extern unsigned int sysctl_sched_uclamp_util_max;
    > --- a/kernel/sched/deadline.c
    > +++ b/kernel/sched/deadline.c
    > @@ -2597,6 +2597,14 @@ void __getparam_dl(struct task_struct *p
    > }
    >
    > /*
    > + * Default limits for DL period; on the top end we guard against small util
    > + * tasks still getting rediculous long effective runtimes, on the bottom end we
    > + * guard against timer DoS.
    > + */
    > +unsigned int sysctl_sched_dl_period_max = 1 << 22; /* ~4 seconds */
    > +unsigned int sysctl_sched_dl_period_min = 100; /* 100 us */
    > +
    > +/*
    > * This function validates the new parameters of a -deadline task.
    > * We ask for the deadline not being zero, and greater or equal
    > * than the runtime, as well as the period of being zero or
    > @@ -2608,6 +2616,8 @@ void __getparam_dl(struct task_struct *p
    > */
    > bool __checkparam_dl(const struct sched_attr *attr)
    > {
    > + u64 period, max, min;
    > +
    > /* special dl tasks don't actually use any parameter */
    > if (attr->sched_flags & SCHED_FLAG_SUGOV)
    > return true;
    > @@ -2631,12 +2641,21 @@ bool __checkparam_dl(const struct sched_
    > attr->sched_period & (1ULL << 63))
    > return false;
    >
    > + period = attr->sched_period;
    > + if (!period)
    > + period = attr->sched_deadline;
    > +
    > /* runtime <= deadline <= period (if period != 0) */
    > - if ((attr->sched_period != 0 &&
    > - attr->sched_period < attr->sched_deadline) ||
    > + if (period < attr->sched_deadline ||
    > attr->sched_deadline < attr->sched_runtime)
    > return false;
    >
    > + max = (u64)READ_ONCE(sysctl_sched_dl_period_max) * NSEC_PER_USEC;
    > + min = (u64)READ_ONCE(sysctl_sched_dl_period_min) * NSEC_PER_USEC;
    > +
    > + if (period < min || period > max)
    > + return false;
    > +
    > return true;
    > }
    >
    > --- a/kernel/sysctl.c
    > +++ b/kernel/sysctl.c
    > @@ -443,6 +443,20 @@ static struct ctl_table kern_table[] = {
    > .proc_handler = sched_rt_handler,
    > },
    > {
    > + .procname = "sched_deadline_period_max_us",
    > + .data = &sysctl_sched_dl_period_max,
    > + .maxlen = sizeof(unsigned int),
    > + .mode = 0644,
    > + .proc_handler = proc_dointvec,
    > + },
    > + {
    > + .procname = "sched_deadline_period_min_us",
    > + .data = &sysctl_sched_dl_period_min,
    > + .maxlen = sizeof(unsigned int),
    > + .mode = 0644,
    > + .proc_handler = proc_dointvec,
    > + },
    > + {
    > .procname = "sched_rr_timeslice_ms",
    > .data = &sysctl_sched_rr_timeslice,
    > .maxlen = sizeof(int),
    >
    >

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