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    Subject[PATCH tip/core/rcu 20/20] rcu: Create RCU-specific workqueues with rescuers
    Date
    RCU's expedited grace periods can participate in out-of-memory deadlocks
    due to all available system_wq kthreads being blocked and there not being
    memory available to create more. This commit prevents such deadlocks
    by allocating an RCU-specific workqueue_struct at early boot time, and
    providing it with a rescuer to ensure forward progress. This uses the
    shiny new init_rescuer() function provided by Tejun (but indirectly).

    This commit also causes SRCU to use this new RCU-specific
    workqueue_struct. Note that SRCU's use of workqueues never blocks them
    waiting for readers, so this should be safe from a forward-progress
    viewpoint. Note that this moves SRCU from system_power_efficient_wq
    to a normal workqueue. In the unlikely event that this results in
    measurable degradation, a separate power-efficient workqueue will be
    creates for SRCU.

    Reported-by: Prateek Sood <prsood@codeaurora.org>
    Reported-by: Tejun Heo <tj@kernel.org>
    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Acked-by: Tejun Heo <tj@kernel.org>
    ---
    kernel/rcu/rcu.h | 1 +
    kernel/rcu/srcutree.c | 8 +++-----
    kernel/rcu/tree.c | 6 ++++++
    kernel/rcu/tree_exp.h | 2 +-
    4 files changed, 11 insertions(+), 6 deletions(-)

    diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
    index 1c868bcfd705..7a693e31184a 100644
    --- a/kernel/rcu/rcu.h
    +++ b/kernel/rcu/rcu.h
    @@ -485,6 +485,7 @@ void show_rcu_gp_kthreads(void);
    void rcu_force_quiescent_state(void);
    void rcu_bh_force_quiescent_state(void);
    void rcu_sched_force_quiescent_state(void);
    +extern struct workqueue_struct *rcu_gp_wq;
    #endif /* #else #ifdef CONFIG_TINY_RCU */

    #ifdef CONFIG_RCU_NOCB_CPU
    diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
    index 045b559b9f22..743d18379256 100644
    --- a/kernel/rcu/srcutree.c
    +++ b/kernel/rcu/srcutree.c
    @@ -492,8 +492,7 @@ static bool srcu_queue_delayed_work_on(int cpu, struct workqueue_struct *wq,
    */
    static void srcu_schedule_cbs_sdp(struct srcu_data *sdp, unsigned long delay)
    {
    - srcu_queue_delayed_work_on(sdp->cpu, system_power_efficient_wq,
    - &sdp->work, delay);
    + srcu_queue_delayed_work_on(sdp->cpu, rcu_gp_wq, &sdp->work, delay);
    }

    /*
    @@ -691,8 +690,7 @@ static void srcu_funnel_gp_start(struct srcu_struct *sp, struct srcu_data *sdp,
    rcu_seq_state(sp->srcu_gp_seq) == SRCU_STATE_IDLE) {
    WARN_ON_ONCE(ULONG_CMP_GE(sp->srcu_gp_seq, sp->srcu_gp_seq_needed));
    srcu_gp_start(sp);
    - queue_delayed_work(system_power_efficient_wq, &sp->work,
    - srcu_get_delay(sp));
    + queue_delayed_work(rcu_gp_wq, &sp->work, srcu_get_delay(sp));
    }
    spin_unlock_irqrestore_rcu_node(sp, flags);
    }
    @@ -1225,7 +1223,7 @@ static void srcu_reschedule(struct srcu_struct *sp, unsigned long delay)
    spin_unlock_irq_rcu_node(sp);

    if (pushgp)
    - queue_delayed_work(system_power_efficient_wq, &sp->work, delay);
    + queue_delayed_work(rcu_gp_wq, &sp->work, delay);
    }

    /*
    diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
    index 99d404c6bbbb..2a734692a581 100644
    --- a/kernel/rcu/tree.c
    +++ b/kernel/rcu/tree.c
    @@ -4167,6 +4167,8 @@ static void __init rcu_dump_rcu_node_tree(struct rcu_state *rsp)
    pr_cont("\n");
    }

    +struct workqueue_struct *rcu_gp_wq;
    +
    void __init rcu_init(void)
    {
    int cpu;
    @@ -4193,6 +4195,10 @@ void __init rcu_init(void)
    rcu_cpu_starting(cpu);
    rcutree_online_cpu(cpu);
    }
    +
    + /* Create workqueue for expedited GPs and for Tree SRCU. */
    + rcu_gp_wq = alloc_workqueue("rcu_gp", WQ_MEM_RECLAIM, 0);
    + WARN_ON(!rcu_gp_wq);
    }

    #include "tree_exp.h"
    diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
    index 6ad87642f44a..f72eefab8543 100644
    --- a/kernel/rcu/tree_exp.h
    +++ b/kernel/rcu/tree_exp.h
    @@ -626,7 +626,7 @@ static void _synchronize_rcu_expedited(struct rcu_state *rsp,
    rew.rew_rsp = rsp;
    rew.rew_s = s;
    INIT_WORK_ONSTACK(&rew.rew_work, wait_rcu_exp_gp);
    - schedule_work(&rew.rew_work);
    + queue_work(rcu_gp_wq, &rew.rew_work);
    }

    /* Wait for expedited grace period to complete. */
    --
    2.5.2
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