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Message-ID: <20201128021609.GX1437@paulmck-ThinkPad-P72>
Date: Fri, 27 Nov 2020 18:16:09 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Neeraj Upadhyay <neeraju@...eaurora.org>
Cc: rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
kernel-team@...com, mingo@...nel.org, jiangshanlai@...il.com,
akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
fweisbec@...il.com, oleg@...hat.com, joel@...lfernandes.org,
kent.overstreet@...il.com
Subject: Re: [PATCH v2 tip/core/rcu 1/6] srcu: Make Tiny SRCU use multi-bit
grace-period counter
On Wed, Nov 25, 2020 at 10:03:26AM +0530, Neeraj Upadhyay wrote:
>
>
> On 11/24/2020 10:48 AM, Neeraj Upadhyay wrote:
> >
> >
> > On 11/24/2020 1:25 AM, Paul E. McKenney wrote:
> > > On Mon, Nov 23, 2020 at 10:01:13AM +0530, Neeraj Upadhyay wrote:
> > > > On 11/21/2020 6:29 AM, paulmck@...nel.org wrote:
> > > > > From: "Paul E. McKenney" <paulmck@...nel.org>
> > > > >
> > > > > There is a need for a polling interface for SRCU grace periods. This
> > > > > polling needs to distinguish between an SRCU instance being idle on the
> > > > > one hand or in the middle of a grace period on the other. This commit
> > > > > therefore converts the Tiny SRCU srcu_struct structure's srcu_idx from
> > > > > a defacto boolean to a free-running counter, using the bottom bit to
> > > > > indicate that a grace period is in progress. The second-from-bottom
> > > > > bit is thus used as the index returned by srcu_read_lock().
> > > > >
> > > > > Link:
> > > > > https://lore.kernel.org/rcu/20201112201547.GF3365678@moria.home.lan/
> > > > > Reported-by: Kent Overstreet <kent.overstreet@...il.com>
> > > > > [ paulmck: Fix __srcu_read_lock() idx computation Neeraj per
> > > > > Upadhyay. ]
> > > > > Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> > > > > ---
> > > > > include/linux/srcutiny.h | 4 ++--
> > > > > kernel/rcu/srcutiny.c | 5 +++--
> > > > > 2 files changed, 5 insertions(+), 4 deletions(-)
> > > > >
> > > > > diff --git a/include/linux/srcutiny.h b/include/linux/srcutiny.h
> > > > > index 5a5a194..d9edb67 100644
> > > > > --- a/include/linux/srcutiny.h
> > > > > +++ b/include/linux/srcutiny.h
> > > > > @@ -15,7 +15,7 @@
> > > > > struct srcu_struct {
> > > > > short srcu_lock_nesting[2]; /* srcu_read_lock()
> > > > > nesting depth. */
> > > > > - short srcu_idx; /* Current reader array element. */
> > > > > + unsigned short srcu_idx; /* Current reader array
> > > > > element in bit 0x2. */
> > > > > u8 srcu_gp_running; /* GP workqueue running? */
> > > > > u8 srcu_gp_waiting; /* GP waiting for readers? */
> > > > > struct swait_queue_head srcu_wq;
> > > > > @@ -59,7 +59,7 @@ static inline int __srcu_read_lock(struct
> > > > > srcu_struct *ssp)
> > > > > {
> > > > > int idx;
> > > > > - idx = READ_ONCE(ssp->srcu_idx);
> > > > > + idx = ((READ_ONCE(ssp->srcu_idx) + 1) & 0x2) >> 1;
> > > > > WRITE_ONCE(ssp->srcu_lock_nesting[idx],
> > > > > ssp->srcu_lock_nesting[idx] + 1);
> > > > > return idx;
> > > > > }
> > > >
> > > > Need change in idx calcultion in srcu_torture_stats_print() ?
> > > >
> > > > static inline void srcu_torture_stats_print(struct srcu_struct *ssp,
> > > > idx = READ_ONCE(ssp->srcu_idx) & 0x1;
> > >
> > > Excellent point! It should match the calculation in __srcu_read_lock(),
> > > shouldn't it? I have updated this, thank you!
> > >
> > > Thanx, Paul
> > >
> >
> > Updated version looks good!
> >
> >
> > Thanks
> > Neeraj
> >
>
> For the version in rcu -dev:
>
> Reviewed-by: Neeraj Upadhyay <neeraju@...eaurora.org>
I applied all of these, thank you very much!
> Only minor point which I have is, the idx calculation can be made an inline
> func (though srcu_drive_gp() does not require a READ_ONCE for ->srcu_idx):
>
> __srcu_read_lock() and srcu_torture_stats_print() are using
>
> idx = ((READ_ONCE(ssp->srcu_idx) + 1) & 0x2) >> 1;
>
> whereas srcu_drive_gp() uses:
>
> idx = (ssp->srcu_idx & 0x2) / 2;
They do work on different elements of the various arrays. Or do you
believe that the srcu_drive_gp() use needs adjusting?
Either way, the overhead of READ_ONCE() is absolutely not at all
a problem. Would you like to put together a patch so that I can see
exactly what you are suggesting?
Thanx, Paul
> Thanks
> Neeraj
>
> > > > Thanks
> > > > Neeraj
> > > >
> > > > > diff --git a/kernel/rcu/srcutiny.c b/kernel/rcu/srcutiny.c
> > > > > index 6208c1d..5598cf6 100644
> > > > > --- a/kernel/rcu/srcutiny.c
> > > > > +++ b/kernel/rcu/srcutiny.c
> > > > > @@ -124,11 +124,12 @@ void srcu_drive_gp(struct work_struct *wp)
> > > > > ssp->srcu_cb_head = NULL;
> > > > > ssp->srcu_cb_tail = &ssp->srcu_cb_head;
> > > > > local_irq_enable();
> > > > > - idx = ssp->srcu_idx;
> > > > > - WRITE_ONCE(ssp->srcu_idx, !ssp->srcu_idx);
> > > > > + idx = (ssp->srcu_idx & 0x2) / 2;
> > > > > + WRITE_ONCE(ssp->srcu_idx, ssp->srcu_idx + 1);
> > > > > WRITE_ONCE(ssp->srcu_gp_waiting, true); /*
> > > > > srcu_read_unlock() wakes! */
> > > > > swait_event_exclusive(ssp->srcu_wq,
> > > > > !READ_ONCE(ssp->srcu_lock_nesting[idx]));
> > > > > WRITE_ONCE(ssp->srcu_gp_waiting, false); /*
> > > > > srcu_read_unlock() cheap. */
> > > > > + WRITE_ONCE(ssp->srcu_idx, ssp->srcu_idx + 1);
> > > > > /* Invoke the callbacks we removed above. */
> > > > > while (lh) {
> > > > >
> > > >
> > > > --
> > > > QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is
> > > > a member of
> > > > the Code Aurora Forum, hosted by The Linux Foundation
> >
>
> --
> QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member of
> the Code Aurora Forum, hosted by The Linux Foundation
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