Re: [PATCH v2 tip/core/rcu 1/6] srcu: Make Tiny SRCU use multi-bit grace-period counter

["Paul E. McKenney" <paulmck@kernel.org>]

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@kernel.org wrote:
> > > > > From: "Paul E. McKenney" <paulmck@kernel.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@gmail.com>
> > > > > [ paulmck: Fix __srcu_read_lock() idx computation Neeraj per
> > > > > Upadhyay. ]
> > > > > Signed-off-by: Paul E. McKenney <paulmck@kernel.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@codeaurora.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