Messages in this thread |  | | Subject | Re: [PATCH v2 tip/core/rcu 1/6] srcu: Make Tiny SRCU use multi-bit grace-period counter | From | Neeraj Upadhyay <> | Date | Sat, 28 Nov 2020 09:42:55 +0530 |
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On 11/28/2020 7:46 AM, Paul E. McKenney wrote: > 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! >
Welcome :)
>> 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?
My bad, I missed that they are using different elements of array. Please ignore this comment.
Thanks Neeraj
> > 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
-- 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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