Re: [PATCH v4 3/3] rcu: Use _full() API to debug synchronize_rcu()

[Uladzislau Rezki <urezki@gmail.com>]

On Fri, Feb 28, 2025 at 05:36:47PM +0100, Uladzislau Rezki wrote:
> On Fri, Feb 28, 2025 at 07:41:40AM -0800, Paul E. McKenney wrote:
> > On Thu, Feb 27, 2025 at 06:44:15PM +0100, Uladzislau Rezki wrote:
> > > On Thu, Feb 27, 2025 at 09:26:40AM -0800, Paul E. McKenney wrote:
> > > > On Thu, Feb 27, 2025 at 09:12:39AM -0800, Boqun Feng wrote:
> > > > > Hi Ulad,
> > > > > 
> > > > > I put these three patches into next (and misc.2025.02.27a) for some
> > > > > testing, hopefully it all goes well and they can make it v6.15.
> > > > > 
> > > > > A few tag changed below:
> > > > > 
> > > > > On Thu, Feb 27, 2025 at 02:16:13PM +0100, Uladzislau Rezki (Sony) wrote:
> > > > > > Switch for using of get_state_synchronize_rcu_full() and
> > > > > > poll_state_synchronize_rcu_full() pair to debug a normal
> > > > > > synchronize_rcu() call.
> > > > > > 
> > > > > > Just using "not" full APIs to identify if a grace period is
> > > > > > passed or not might lead to a false-positive kernel splat.
> > > > > > 
> > > > > > It can happen, because get_state_synchronize_rcu() compresses
> > > > > > both normal and expedited states into one single unsigned long
> > > > > > value, so a poll_state_synchronize_rcu() can miss GP-completion
> > > > > > when synchronize_rcu()/synchronize_rcu_expedited() concurrently
> > > > > > run.
> > > > > > 
> > > > > > To address this, switch to poll_state_synchronize_rcu_full() and
> > > > > > get_state_synchronize_rcu_full() APIs, which use separate variables
> > > > > > for expedited and normal states.
> > > > > > 
> > > > > > Link: https://lore.kernel.org/lkml/Z5ikQeVmVdsWQrdD@pc636/T/
> > > > > 
> > > > > I switch this into "Closes:" per checkpatch.
> > > > > 
> > > > > > Fixes: 988f569ae041 ("rcu: Reduce synchronize_rcu() latency")
> > > > > > Reported-by: cheung wall <zzqq0103.hey@gmail.com>
> > > > > > Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> > > > > 
> > > > > You seem to forget add Paul's Reviewed-by, so I add it in rcu/next.
> > > > > Would you or Paul double-check the Reviewed-by should be here?
> > > > 
> > > > I am good with keeping my Reviewed-by tags.
> > > > 
> > > Thanks Paul!
> > 
> > Except that I got this from overnight testing of rcu/dev on the shared
> > RCU tree:
> > 
> > WARNING: CPU: 5 PID: 14 at kernel/rcu/tree.c:1636 rcu_sr_normal_complete+0x5c/0x80
> > 
> > I see this only on TREE05.  Which should not be too surprising, given
> > that this is the scenario that tests it.  It happened within five minutes
> > on all 14 of the TREE05 runs.
> > 
> Hm.. This is not fun. I tested this on my system and i did not manage to
> trigger this whereas you do. Something is wrong.
>
We have below code to start a new GP, if we detect that processing is
starved:

<snip>
/*
 * The "start_new_poll" is set to true, only when this GP is not able
 * to handle anything and there are outstanding users. It happens when
 * the rcu_sr_normal_gp_init() function was not able to insert a dummy
 * separator to the llist, because there were no left any dummy-nodes.
 *
 * Number of dummy-nodes is fixed, it could be that we are run out of
 * them, if so we start a new pool request to repeat a try. It is rare
 * and it means that a system is doing a slow processing of callbacks.
 */
  if (start_new_poll)
    (void) start_poll_synchronize_rcu();
<snip>

we do not use a _full() version, since we need to inform rcu-gp-kthread
to initiate a new GP.

Any thoughts?

--
Uladzislau Rezki