RE: [PATCH for-next v3 1/7] RDMA/rxe: Convert triple tasklets to use workqueue

From: Daisuke Matsuda (Fujitsu)
Date: Thu Jan 05 2023 - 21:27:58 EST

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On Thu, Dec 29, 2022 1:56 AM Bob Pearson wrote:
>
> On 12/23/22 00:51, Daisuke Matsuda wrote:
> > In order to implement On-Demand Paging on the rxe driver, triple tasklets
> > (requester, responder, and completer) must be allowed to sleep so that they
> > can trigger page fault when pages being accessed are not present.
> >
> > This patch replaces the tasklets with a workqueue, but still allows direct-
> > call of works from softirq context if it is obvious that MRs are not going
> > to be accessed and there is no work being processed in the workqueue.
>
> There are already at least two patch sets that do this waiting to get upstream.
> Bob

I wrote my intention at the first part of the cover letter.
Cf. https://lore.kernel.org/lkml/cover.1671772917.git.matsuda-daisuke@xxxxxxxxxxx/

Your patch set introduces a soft lockup issue. It would take much more time to find
the root cause than to simply convert the tasklets to a workqueue with this patch.
My ODP patches have been stuck for almost 4 months because of this issue, and I am
not willing to wait any longer.

Daisuke

>
> >
> > As counterparts to tasklet_disable() and tasklet_enable() are missing for
> > workqueues, an atomic value is introduced to prevent work items from being
> > scheduled while qp reset is in progress.
> >
> > The way to initialize/destroy workqueue is picked up from the
> > implementation of Ian Ziemba and Bob Pearson at HPE.
> >
> > Link: https://lore.kernel.org/all/20221018043345.4033-1-rpearsonhpe@xxxxxxxxx/
> > Signed-off-by: Daisuke Matsuda <matsuda-daisuke@xxxxxxxxxxx>
> > ---
> > drivers/infiniband/sw/rxe/rxe.c | 9 ++++-
> > drivers/infiniband/sw/rxe/rxe_comp.c | 2 +-
> > drivers/infiniband/sw/rxe/rxe_param.h | 2 +-
> > drivers/infiniband/sw/rxe/rxe_qp.c | 2 +-
> > drivers/infiniband/sw/rxe/rxe_req.c | 2 +-
> > drivers/infiniband/sw/rxe/rxe_resp.c | 2 +-
> > drivers/infiniband/sw/rxe/rxe_task.c | 52 ++++++++++++++++++++-------
> > drivers/infiniband/sw/rxe/rxe_task.h | 15 ++++++--
> > 8 files changed, 65 insertions(+), 21 deletions(-)
> >
> > diff --git a/drivers/infiniband/sw/rxe/rxe.c b/drivers/infiniband/sw/rxe/rxe.c
> > index 136c2efe3466..3c7e42e5b0c7 100644
> > --- a/drivers/infiniband/sw/rxe/rxe.c
> > +++ b/drivers/infiniband/sw/rxe/rxe.c
> > @@ -210,10 +210,16 @@ static int __init rxe_module_init(void)
> > {
> > int err;
> >
> > - err = rxe_net_init();
> > + err = rxe_alloc_wq();
> > if (err)
> > return err;
> >
> > + err = rxe_net_init();
> > + if (err) {
> > + rxe_destroy_wq();
> > + return err;
> > + }
> > +
> > rdma_link_register(&rxe_link_ops);
> > pr_info("loaded\n");
> > return 0;
> > @@ -224,6 +230,7 @@ static void __exit rxe_module_exit(void)
> > rdma_link_unregister(&rxe_link_ops);
> > ib_unregister_driver(RDMA_DRIVER_RXE);
> > rxe_net_exit();
> > + rxe_destroy_wq();
> >
> > pr_info("unloaded\n");
> > }
> > diff --git a/drivers/infiniband/sw/rxe/rxe_comp.c b/drivers/infiniband/sw/rxe/rxe_comp.c
> > index 20737fec392b..046bbacce37c 100644
> > --- a/drivers/infiniband/sw/rxe/rxe_comp.c
> > +++ b/drivers/infiniband/sw/rxe/rxe_comp.c
> > @@ -773,7 +773,7 @@ int rxe_completer(void *arg)
> > }
> >
> > /* A non-zero return value will cause rxe_do_task to
> > - * exit its loop and end the tasklet. A zero return
> > + * exit its loop and end the work item. A zero return
> > * will continue looping and return to rxe_completer
> > */
> > done:
> > diff --git a/drivers/infiniband/sw/rxe/rxe_param.h b/drivers/infiniband/sw/rxe/rxe_param.h
> > index a754fc902e3d..bd8050e99d6b 100644
> > --- a/drivers/infiniband/sw/rxe/rxe_param.h
> > +++ b/drivers/infiniband/sw/rxe/rxe_param.h
> > @@ -112,7 +112,7 @@ enum rxe_device_param {
> > RXE_INFLIGHT_SKBS_PER_QP_HIGH = 64,
> > RXE_INFLIGHT_SKBS_PER_QP_LOW = 16,
> >
> > - /* Max number of interations of each tasklet
> > + /* Max number of interations of each work item
> > * before yielding the cpu to let other
> > * work make progress
> > */
> > diff --git a/drivers/infiniband/sw/rxe/rxe_qp.c b/drivers/infiniband/sw/rxe/rxe_qp.c
> > index ab72db68b58f..e033b2449dfe 100644
> > --- a/drivers/infiniband/sw/rxe/rxe_qp.c
> > +++ b/drivers/infiniband/sw/rxe/rxe_qp.c
> > @@ -471,7 +471,7 @@ int rxe_qp_chk_attr(struct rxe_dev *rxe, struct rxe_qp *qp,
> > /* move the qp to the reset state */
> > static void rxe_qp_reset(struct rxe_qp *qp)
> > {
> > - /* stop tasks from running */
> > + /* flush workqueue and stop tasks from running */
> > rxe_disable_task(&qp->resp.task);
> >
> > /* stop request/comp */
> > diff --git a/drivers/infiniband/sw/rxe/rxe_req.c b/drivers/infiniband/sw/rxe/rxe_req.c
> > index 899c8779f800..2bcd287a2c3b 100644
> > --- a/drivers/infiniband/sw/rxe/rxe_req.c
> > +++ b/drivers/infiniband/sw/rxe/rxe_req.c
> > @@ -830,7 +830,7 @@ int rxe_requester(void *arg)
> > update_state(qp, &pkt);
> >
> > /* A non-zero return value will cause rxe_do_task to
> > - * exit its loop and end the tasklet. A zero return
> > + * exit its loop and end the work item. A zero return
> > * will continue looping and return to rxe_requester
> > */
> > done:
> > diff --git a/drivers/infiniband/sw/rxe/rxe_resp.c b/drivers/infiniband/sw/rxe/rxe_resp.c
> > index c74972244f08..d9134a00a529 100644
> > --- a/drivers/infiniband/sw/rxe/rxe_resp.c
> > +++ b/drivers/infiniband/sw/rxe/rxe_resp.c
> > @@ -1691,7 +1691,7 @@ int rxe_responder(void *arg)
> > }
> >
> > /* A non-zero return value will cause rxe_do_task to
> > - * exit its loop and end the tasklet. A zero return
> > + * exit its loop and end the work item. A zero return
> > * will continue looping and return to rxe_responder
> > */
> > done:
> > diff --git a/drivers/infiniband/sw/rxe/rxe_task.c b/drivers/infiniband/sw/rxe/rxe_task.c
> > index 60b90e33a884..b96f72aa9005 100644
> > --- a/drivers/infiniband/sw/rxe/rxe_task.c
> > +++ b/drivers/infiniband/sw/rxe/rxe_task.c
> > @@ -6,6 +6,22 @@
> >
> > #include "rxe.h"
> >
> > +static struct workqueue_struct *rxe_wq;
> > +
> > +int rxe_alloc_wq(void)
> > +{
> > + rxe_wq = alloc_workqueue("rxe_wq", WQ_CPU_INTENSIVE, WQ_MAX_ACTIVE);
> > + if (!rxe_wq)
> > + return -ENOMEM;
> > +
> > + return 0;
> > +}
> > +
> > +void rxe_destroy_wq(void)
> > +{
> > + destroy_workqueue(rxe_wq);
> > +}
> > +
> > int __rxe_do_task(struct rxe_task *task)
> >
> > {
> > @@ -24,11 +40,11 @@ int __rxe_do_task(struct rxe_task *task)
> > * a second caller finds the task already running
> > * but looks just after the last call to func
> > */
> > -static void do_task(struct tasklet_struct *t)
> > +static void do_task(struct work_struct *w)
> > {
> > int cont;
> > int ret;
> > - struct rxe_task *task = from_tasklet(task, t, tasklet);
> > + struct rxe_task *task = container_of(w, typeof(*task), work);
> > struct rxe_qp *qp = (struct rxe_qp *)task->arg;
> > unsigned int iterations = RXE_MAX_ITERATIONS;
> >
> > @@ -64,10 +80,10 @@ static void do_task(struct tasklet_struct *t)
> > } else if (iterations--) {
> > cont = 1;
> > } else {
> > - /* reschedule the tasklet and exit
> > + /* reschedule the work item and exit
> > * the loop to give up the cpu
> > */
> > - tasklet_schedule(&task->tasklet);
> > + queue_work(task->workq, &task->work);
> > task->state = TASK_STATE_START;
> > }
> > break;
> > @@ -97,7 +113,8 @@ int rxe_init_task(struct rxe_task *task, void *arg, int (*func)(void *))
> > task->func = func;
> > task->destroyed = false;
> >
> > - tasklet_setup(&task->tasklet, do_task);
> > + INIT_WORK(&task->work, do_task);
> > + task->workq = rxe_wq;
> >
> > task->state = TASK_STATE_START;
> > spin_lock_init(&task->lock);
> > @@ -111,17 +128,16 @@ void rxe_cleanup_task(struct rxe_task *task)
> >
> > /*
> > * Mark the task, then wait for it to finish. It might be
> > - * running in a non-tasklet (direct call) context.
> > + * running in a non-workqueue (direct call) context.
> > */
> > task->destroyed = true;
> > + flush_workqueue(task->workq);
> >
> > do {
> > spin_lock_bh(&task->lock);
> > idle = (task->state == TASK_STATE_START);
> > spin_unlock_bh(&task->lock);
> > } while (!idle);
> > -
> > - tasklet_kill(&task->tasklet);
> > }
> >
> > void rxe_run_task(struct rxe_task *task)
> > @@ -129,7 +145,7 @@ void rxe_run_task(struct rxe_task *task)
> > if (task->destroyed)
> > return;
> >
> > - do_task(&task->tasklet);
> > + do_task(&task->work);
> > }
> >
> > void rxe_sched_task(struct rxe_task *task)
> > @@ -137,15 +153,27 @@ void rxe_sched_task(struct rxe_task *task)
> > if (task->destroyed)
> > return;
> >
> > - tasklet_schedule(&task->tasklet);
> > + /*
> > + * busy-loop while qp reset is in progress.
> > + * This may be called from softirq context and thus cannot sleep.
> > + */
> > + while (atomic_read(&task->suspended))
> > + cpu_relax();
> > +
> > + queue_work(task->workq, &task->work);
> > }
> >
> > void rxe_disable_task(struct rxe_task *task)
> > {
> > - tasklet_disable(&task->tasklet);
> > + /* Alternative to tasklet_disable() */
> > + atomic_inc(&task->suspended);
> > + smp_mb__after_atomic();
> > + flush_workqueue(task->workq);
> > }
> >
> > void rxe_enable_task(struct rxe_task *task)
> > {
> > - tasklet_enable(&task->tasklet);
> > + /* Alternative to tasklet_enable() */
> > + smp_mb__before_atomic();
> > + atomic_dec(&task->suspended);
> > }
> > diff --git a/drivers/infiniband/sw/rxe/rxe_task.h b/drivers/infiniband/sw/rxe/rxe_task.h
> > index 7b88129702ac..9aa3f236e886 100644
> > --- a/drivers/infiniband/sw/rxe/rxe_task.h
> > +++ b/drivers/infiniband/sw/rxe/rxe_task.h
> > @@ -19,15 +19,22 @@ enum {
> > * called again.
> > */
> > struct rxe_task {
> > - struct tasklet_struct tasklet;
> > + struct workqueue_struct *workq;
> > + struct work_struct work;
> > int state;
> > spinlock_t lock;
> > void *arg;
> > int (*func)(void *arg);
> > int ret;
> > bool destroyed;
> > + /* used to {dis, en}able per-qp work items */
> > + atomic_t suspended;
> > };
> >
> > +int rxe_alloc_wq(void);
> > +
> > +void rxe_destroy_wq(void);
> > +
> > /*
> > * init rxe_task structure
> > * arg => parameter to pass to fcn
> > @@ -40,18 +47,20 @@ void rxe_cleanup_task(struct rxe_task *task);
> >
> > /*
> > * raw call to func in loop without any checking
> > - * can call when tasklets are disabled
> > + * can call when tasks are suspended
> > */
> > int __rxe_do_task(struct rxe_task *task);
> >
> > +/* run a task without scheduling */
> > void rxe_run_task(struct rxe_task *task);
> >
> > +/* schedule a task into workqueue */
> > void rxe_sched_task(struct rxe_task *task);
> >
> > /* keep a task from scheduling */
> > void rxe_disable_task(struct rxe_task *task);
> >
> > -/* allow task to run */
> > +/* allow a task to run again */
> > void rxe_enable_task(struct rxe_task *task);
> >
> > #endif /* RXE_TASK_H */

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