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Subject: Re: [PATCH v2 tip/core/rcu 4/6] srcu: Provide polling interfaces for
 Tiny SRCU grace periods
To:     paulmck@kernel.org
Cc:     rcu@vger.kernel.org, linux-kernel@vger.kernel.org,
        kernel-team@fb.com, mingo@kernel.org, jiangshanlai@gmail.com,
        akpm@linux-foundation.org, mathieu.desnoyers@efficios.com,
        josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org,
        rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com,
        fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org,
        kent.overstreet@gmail.com
References: <@@@> <20201121005919.17152-4-paulmck@kernel.org>
 <2d210fa7-3484-9cee-862b-a8f15fee8c8e@codeaurora.org>
 <20201123211250.GF1437@paulmck-ThinkPad-P72>
From:   Neeraj Upadhyay <neeraju@codeaurora.org>
Message-ID: <9ebe7c82-6fc2-a759-dde8-eec90b483735@codeaurora.org>
Date:   Tue, 24 Nov 2020 10:44:24 +0530
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On 11/24/2020 2:42 AM, Paul E. McKenney wrote:
> On Mon, Nov 23, 2020 at 10:13:13AM +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, so this commit supplies get_state_synchronize_srcu(),
>>> start_poll_synchronize_srcu(), and poll_state_synchronize_srcu() for this
>>> purpose.  The first can be used if future grace periods are inevitable
>>> (perhaps due to a later call_srcu() invocation), the second if future
>>> grace periods might not otherwise happen, and the third to check if a
>>> grace period has elapsed since the corresponding call to either of the
>>> first two.
>>>
>>> As with get_state_synchronize_rcu() and cond_synchronize_rcu(),
>>> the return value from either get_state_synchronize_srcu() or
>>> start_poll_synchronize_srcu() must be passed in to a later call to
>>> poll_state_synchronize_srcu().
>>>
>>> Link: https://lore.kernel.org/rcu/20201112201547.GF3365678@moria.home.lan/
>>> Reported-by: Kent Overstreet <kent.overstreet@gmail.com>
>>> [ paulmck: Add EXPORT_SYMBOL_GPL() per kernel test robot feedback. ]
>>> [ paulmck: Apply feedback from Neeraj Upadhyay. ]
>>> Link: https://lore.kernel.org/lkml/20201117004017.GA7444@paulmck-ThinkPad-P72/
>>> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
>>> ---
>>>    include/linux/rcupdate.h |  2 ++
>>>    include/linux/srcu.h     |  3 +++
>>>    include/linux/srcutiny.h |  1 +
>>>    kernel/rcu/srcutiny.c    | 52 ++++++++++++++++++++++++++++++++++++++++++++++--
>>>    4 files changed, 56 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
>>> index de08264..e09c0d8 100644
>>> --- a/include/linux/rcupdate.h
>>> +++ b/include/linux/rcupdate.h
>>> @@ -33,6 +33,8 @@
>>>    #define ULONG_CMP_GE(a, b)	(ULONG_MAX / 2 >= (a) - (b))
>>>    #define ULONG_CMP_LT(a, b)	(ULONG_MAX / 2 < (a) - (b))
>>>    #define ulong2long(a)		(*(long *)(&(a)))
>>> +#define USHORT_CMP_GE(a, b)	(USHRT_MAX / 2 >= (unsigned short)((a) - (b)))
>>> +#define USHORT_CMP_LT(a, b)	(USHRT_MAX / 2 < (unsigned short)((a) - (b)))
>>>    /* Exported common interfaces */
>>>    void call_rcu(struct rcu_head *head, rcu_callback_t func);
>>> diff --git a/include/linux/srcu.h b/include/linux/srcu.h
>>> index e432cc9..a0895bb 100644
>>> --- a/include/linux/srcu.h
>>> +++ b/include/linux/srcu.h
>>> @@ -60,6 +60,9 @@ void cleanup_srcu_struct(struct srcu_struct *ssp);
>>>    int __srcu_read_lock(struct srcu_struct *ssp) __acquires(ssp);
>>>    void __srcu_read_unlock(struct srcu_struct *ssp, int idx) __releases(ssp);
>>>    void synchronize_srcu(struct srcu_struct *ssp);
>>> +unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp);
>>> +unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp);
>>> +bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie);
>>>    #ifdef CONFIG_DEBUG_LOCK_ALLOC
>>> diff --git a/include/linux/srcutiny.h b/include/linux/srcutiny.h
>>> index d9edb67..c7f0c1f 100644
>>> --- a/include/linux/srcutiny.h
>>> +++ b/include/linux/srcutiny.h
>>> @@ -16,6 +16,7 @@
>>>    struct srcu_struct {
>>>    	short srcu_lock_nesting[2];	/* srcu_read_lock() nesting depth. */
>>>    	unsigned short srcu_idx;	/* Current reader array element in bit 0x2. */
>>> +	unsigned short srcu_idx_max;	/* Furthest future srcu_idx request. */
>>>    	u8 srcu_gp_running;		/* GP workqueue running? */
>>>    	u8 srcu_gp_waiting;		/* GP waiting for readers? */
>>>    	struct swait_queue_head srcu_wq;
>>> diff --git a/kernel/rcu/srcutiny.c b/kernel/rcu/srcutiny.c
>>> index 3bac1db..b073175 100644
>>> --- a/kernel/rcu/srcutiny.c
>>> +++ b/kernel/rcu/srcutiny.c
>>> @@ -34,6 +34,7 @@ static int init_srcu_struct_fields(struct srcu_struct *ssp)
>>>    	ssp->srcu_gp_running = false;
>>>    	ssp->srcu_gp_waiting = false;
>>>    	ssp->srcu_idx = 0;
>>> +	ssp->srcu_idx_max = 0;
>>>    	INIT_WORK(&ssp->srcu_work, srcu_drive_gp);
>>>    	INIT_LIST_HEAD(&ssp->srcu_work.entry);
>>>    	return 0;
>>
>> Minor: cleanup_srcu_struct() can probably have 2 new sanity checks?
>>
>> WARN_ON(ssp->srcu_idx != ssp->srcu_idx_max);
>> WARN_ON(ssp->srcu_idx & 1);
> 
> Good point, added and under test.
> 
>> Thanks
>> Neeraj
>>
>>> @@ -114,7 +115,7 @@ void srcu_drive_gp(struct work_struct *wp)
>>>    	struct srcu_struct *ssp;
>>>    	ssp = container_of(wp, struct srcu_struct, srcu_work);
>>> -	if (ssp->srcu_gp_running || !READ_ONCE(ssp->srcu_cb_head))
>>> +	if (ssp->srcu_gp_running || USHORT_CMP_GE(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
>>>    		return; /* Already running or nothing to do. */
>>>    	/* Remove recently arrived callbacks and wait for readers. */
>>> @@ -147,13 +148,18 @@ void srcu_drive_gp(struct work_struct *wp)
>>>    	 * straighten that out.
>>>    	 */
>>>    	WRITE_ONCE(ssp->srcu_gp_running, false);
>>> -	if (READ_ONCE(ssp->srcu_cb_head))
>>> +	if (USHORT_CMP_GE(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
>>>    		schedule_work(&ssp->srcu_work);
>>>    }
>>>    EXPORT_SYMBOL_GPL(srcu_drive_gp);
>>>    static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
>>>    {
>>> +	unsigned short cookie;
>>> +
>>> +	cookie = get_state_synchronize_srcu(ssp);
>>> +	if (USHORT_CMP_LT(READ_ONCE(ssp->srcu_idx_max), cookie))
>>> +		WRITE_ONCE(ssp->srcu_idx_max, cookie);
>>
>> Minor: Maybe we can return in the else part of USHORT_CMP_LT check, to avoid
>> scheduling work?
> 
> How about like this?

Looks good!


Thanks
Neeraj

> 
> 	static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
> 	{
> 		unsigned short cookie;
> 
> 		cookie = get_state_synchronize_srcu(ssp);
> 		if (USHORT_CMP_GE(READ_ONCE(ssp->srcu_idx_max), cookie))
> 			return;
> 		WRITE_ONCE(ssp->srcu_idx_max, cookie);
> 		if (!READ_ONCE(ssp->srcu_gp_running)) {
> 			if (likely(srcu_init_done))
> 				schedule_work(&ssp->srcu_work);
> 			else if (list_empty(&ssp->srcu_work.entry))
> 				list_add(&ssp->srcu_work.entry, &srcu_boot_list);
> 		}
> 	}
> 
> Testing this next.  ;-)
> 
> 							Thanx, Paul
> 
>> Thanks
>> Neeraj
>>
>>>    	if (!READ_ONCE(ssp->srcu_gp_running)) {
>>>    		if (likely(srcu_init_done))
>>>    			schedule_work(&ssp->srcu_work);
>>> @@ -196,6 +202,48 @@ void synchronize_srcu(struct srcu_struct *ssp)
>>>    }
>>>    EXPORT_SYMBOL_GPL(synchronize_srcu);
>>> +/*
>>> + * get_state_synchronize_srcu - Provide an end-of-grace-period cookie
>>> + */
>>> +unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp)
>>> +{
>>> +	unsigned long ret;
>>> +
>>> +	barrier();
>>> +	ret = (READ_ONCE(ssp->srcu_idx) + 3) & ~0x1;
>>> +	barrier();
>>> +	return ret & USHRT_MAX;
>>> +}
>>> +EXPORT_SYMBOL_GPL(get_state_synchronize_srcu);
>>> +
>>> +/*
>>> + * start_poll_synchronize_srcu - Provide cookie and start grace period
>>> + *
>>> + * The difference between this and get_state_synchronize_srcu() is that
>>> + * this function ensures that the poll_state_synchronize_srcu() will
>>> + * eventually return the value true.
>>> + */
>>> +unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp)
>>> +{
>>> +	unsigned long ret = get_state_synchronize_srcu(ssp);
>>> +
>>> +	srcu_gp_start_if_needed(ssp);
>>> +	return ret;
>>> +}
>>> +EXPORT_SYMBOL_GPL(start_poll_synchronize_srcu);
>>> +
>>> +/*
>>> + * poll_state_synchronize_srcu - Has cookie's grace period ended?
>>> + */
>>> +bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie)
>>> +{
>>> +	bool ret = USHORT_CMP_GE(READ_ONCE(ssp->srcu_idx), cookie);
>>> +
>>> +	barrier();
>>> +	return ret;
>>> +}
>>> +EXPORT_SYMBOL_GPL(poll_state_synchronize_srcu);
>>> +
>>>    /* Lockdep diagnostics.  */
>>>    void __init rcu_scheduler_starting(void)
>>>    {
>>>
>>
>> -- 
>> 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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