Re: [PATCH 2/2] rcu: Allocate WQ with WQ_MEM_RECLAIM bit set

[Uladzislau Rezki <urezki@gmail.com>]

On Wed, Mar 06, 2024 at 12:57:25PM -0500, Joel Fernandes wrote:
> 
> 
> On 3/6/2024 6:56 AM, Uladzislau Rezki wrote:
> > On Wed, Mar 06, 2024 at 10:15:44AM +0800, Z qiang wrote:
> >>>
> >>> synchronize_rcu() users have to be processed regardless
> >>> of memory pressure so our private WQ needs to have at least
> >>> one execution context what WQ_MEM_RECLAIM flag guarantees.
> >>>
> >>> Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> >>> ---
> >>>  kernel/rcu/tree.c | 6 +++++-
> >>>  1 file changed, 5 insertions(+), 1 deletion(-)
> >>>
> >>> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> >>> index 475647620b12..59881a68dd26 100644
> >>> --- a/kernel/rcu/tree.c
> >>> +++ b/kernel/rcu/tree.c
> >>> @@ -1581,6 +1581,7 @@ static void rcu_sr_put_wait_head(struct llist_node *node)
> >>>  /* Disabled by default. */
> >>>  static int rcu_normal_wake_from_gp;
> >>>  module_param(rcu_normal_wake_from_gp, int, 0644);
> >>> +static struct workqueue_struct *sync_wq;
> >>>
> >>>  static void rcu_sr_normal_complete(struct llist_node *node)
> >>>  {
> >>> @@ -1679,7 +1680,7 @@ static void rcu_sr_normal_gp_cleanup(void)
> >>>          * of outstanding users(if still left) and releasing wait-heads
> >>>          * added by rcu_sr_normal_gp_init() call.
> >>>          */
> >>> -       queue_work(system_highpri_wq, &rcu_state.srs_cleanup_work);
> >>> +       queue_work(sync_wq, &rcu_state.srs_cleanup_work);
> >>>  }
> >>>
> >>>  /*
> >>> @@ -5584,6 +5585,9 @@ void __init rcu_init(void)
> >>>         rcu_gp_wq = alloc_workqueue("rcu_gp", WQ_MEM_RECLAIM, 0);
> >>>         WARN_ON(!rcu_gp_wq);
> >>>
> >>> +       sync_wq = alloc_workqueue("sync_wq", WQ_MEM_RECLAIM, 0);
> >>
> >> Why was WQ_HIGHPRI removed?
> >>
> > I would like to check perf. figures with it and send out it as a
> > separate patch if it is worth it.
> 
> I guess one thing to note is that there are also other RCU-related WQ which have
> WQ_MEM_RECLAIM but not WQ_HIGHPRI (such as for expedited RCU, at least some
> configs). So for consistency, this makes sense to me.
> 
> Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org).
> 
Thanks. I will update it with review tag!

--
Uladzislau Rezki