lkml.org 
[lkml]   [2017]   [Sep]   [19]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    Patch in this message
    /
    Date
    From
    SubjectRe: Query regarding synchronize_sched_expedited and resched_cpu
    On Mon, Sep 18, 2017 at 09:24:12AM -0700, Paul E. McKenney wrote:
    > On Mon, Sep 18, 2017 at 12:12:13PM -0400, Steven Rostedt wrote:
    > > On Mon, 18 Sep 2017 09:01:25 -0700
    > > "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
    > >
    > >
    > > > sched: Make resched_cpu() unconditional
    > > >
    > > > The current implementation of synchronize_sched_expedited() incorrectly
    > > > assumes that resched_cpu() is unconditional, which it is not. This means
    > > > that synchronize_sched_expedited() can hang when resched_cpu()'s trylock
    > > > fails as follows (analysis by Neeraj Upadhyay):
    > > >
    > > > o CPU1 is waiting for expedited wait to complete:
    > > > sync_rcu_exp_select_cpus
    > > > rdp->exp_dynticks_snap & 0x1 // returns 1 for CPU5
    > > > IPI sent to CPU5
    > > >
    > > > synchronize_sched_expedited_wait
    > > > ret = swait_event_timeout(
    > > > rsp->expedited_wq,
    > > > sync_rcu_preempt_exp_done(rnp_root),
    > > > jiffies_stall);
    > > >
    > > > expmask = 0x20 , and CPU 5 is in idle path (in cpuidle_enter())
    > > >
    > > > o CPU5 handles IPI and fails to acquire rq lock.
    > > >
    > > > Handles IPI
    > > > sync_sched_exp_handler
    > > > resched_cpu
    > > > returns while failing to try lock acquire rq->lock
    > > > need_resched is not set
    > > >
    > > > o CPU5 calls rcu_idle_enter() and as need_resched is not set, goes to
    > > > idle (schedule() is not called).
    > > >
    > > > o CPU 1 reports RCU stall.
    > > >
    > > > Given that resched_cpu() is used only by RCU, this commit fixes the
    > > > assumption by making resched_cpu() unconditional.
    > >
    > > Probably want to run this with several workloads with lockdep enabled
    > > first.
    >
    > As soon as I work through the backlog of lockdep complaints that
    > appeared in the last merge window... :-(

    And this patch survived all rcutorture scenarios, including those with
    lockdep enabled. There were failures, but these are pre-existing issues
    I am chasing: Lost timeouts on TREE01 and rt_mutex trying to awaken
    an offline CPU in TREE03.

    So I have this one queued. Objections?

    Thanx, Paul

    ------------------------------------------------------------------------

    commit bc43e2e7e08134e6f403ac845edcf4f85668d803
    Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Date: Mon Sep 18 08:54:40 2017 -0700

    sched: Make resched_cpu() unconditional

    The current implementation of synchronize_sched_expedited() incorrectly
    assumes that resched_cpu() is unconditional, which it is not. This means
    that synchronize_sched_expedited() can hang when resched_cpu()'s trylock
    fails as follows (analysis by Neeraj Upadhyay):

    o CPU1 is waiting for expedited wait to complete:
    sync_rcu_exp_select_cpus
    rdp->exp_dynticks_snap & 0x1 // returns 1 for CPU5
    IPI sent to CPU5

    synchronize_sched_expedited_wait
    ret = swait_event_timeout(
    rsp->expedited_wq,
    sync_rcu_preempt_exp_done(rnp_root),
    jiffies_stall);

    expmask = 0x20 , and CPU 5 is in idle path (in cpuidle_enter())

    o CPU5 handles IPI and fails to acquire rq lock.

    Handles IPI
    sync_sched_exp_handler
    resched_cpu
    returns while failing to try lock acquire rq->lock
    need_resched is not set

    o CPU5 calls rcu_idle_enter() and as need_resched is not set, goes to
    idle (schedule() is not called).

    o CPU 1 reports RCU stall.

    Given that resched_cpu() is used only by RCU, this commit fixes the
    assumption by making resched_cpu() unconditional.

    Reported-by: Neeraj Upadhyay <neeraju@codeaurora.org>
    Suggested-by: Neeraj Upadhyay <neeraju@codeaurora.org>
    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Steven Rostedt <rostedt@goodmis.org>

    diff --git a/kernel/sched/core.c b/kernel/sched/core.c
    index cab8c5ec128e..b2281971894c 100644
    --- a/kernel/sched/core.c
    +++ b/kernel/sched/core.c
    @@ -505,8 +505,7 @@ void resched_cpu(int cpu)
    struct rq *rq = cpu_rq(cpu);
    unsigned long flags;

    - if (!raw_spin_trylock_irqsave(&rq->lock, flags))
    - return;
    + raw_spin_lock_irqsave(&rq->lock, flags);
    resched_curr(rq);
    raw_spin_unlock_irqrestore(&rq->lock, flags);
    }
    \
     
     \ /
      Last update: 2017-09-19 17:32    [W:3.566 / U:0.232 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site