Re: [RFC PATCH] SRCU: More efficient reader counts.

From: Mathieu Desnoyers
Date: Fri Nov 18 2016 - 09:26:16 EST


----- On Nov 18, 2016, at 9:08 AM, Paul E. McKenney paulmck@xxxxxxxxxxxxxxxxxx wrote:

> On Thu, Nov 17, 2016 at 02:03:35PM -0800, Lance Roy wrote:
>> SRCU uses two per-cpu counters: a nesting counter to count the number of
>> active critical sections, and a sequence counter to ensure that the nesting
>> counters don't change while they are being added together in
>> srcu_readers_active_idx_check().
>>
>> This patch instead uses per-cpu lock and unlock counters. Because the both
>> counters only increase and srcu_readers_active_idx_check() reads the unlock
>> counter before the lock counter, this achieves the same end without having
>> to increment two different counters in srcu_read_lock(). This also saves a
>> smp_mb() in srcu_readers_active_idx_check().
>>
>> Possible bug: There is no guarantee that the lock counter won't overflow
>> during srcu_readers_active_idx_check(), as there are no memory barriers
>> around srcu_flip() (see comment in srcu_readers_active_idx_check() for
>> details). However, this problem was already present before this patch.
>
> This patch differs from the previous one in a few (good) code-style
> changes, comment changes, and of course the commit log. Good. Once
> discussion converges, I will apply the agreed-upon commit, which might
> well be this one.
>
> However, let's first take a look at the overflow issue.
>
> If a given program could have ULONG_MAX or more readers at any given
> time, there would of course be overflow. However, each read must have
> an srcu_read_lock() outstanding, and the resulting four-byte return
> value must be stored somewhere. Because the full address space is at
> most ULONG_MAX, the maximum number of outstanding readers is at most
> ULONG_MAX/4, even in the degenerate case where a single CPU/task invokes
> srcu_read_lock() in a tight loop. And even this assumes that the entire
> address space can somehow be devoted to srcu_read_lock() return values.
> ULONG_MAX/4 is therefore a hard upper bound on the number of outstanding
> SRCU readers.

The loop taking srcu_read_lock() seems a bit far fetched.

More realistically, we could move this bound even lower
if we can expect each thread to only nest srcu up to a limited
amount (e.g. 128). The realistic limit of number of SRCU readers
then becomes bounded by the maximum number of threads multiplied
by the srcu max nesting count.

>
> Now srcu_readers_active_idx_check() checks for strict equality between
> the number of locks and unlocks, so we can in theory tolerate ULONG_MAX-1
> readers. So, the question is whether ULONG_MAX/4 readers can result
> in the updater seeing ULONG_MAX reads, due to memory misordering and
> other issues.
>
> Because there are no memory barriers surrounding srcu_flip(), the updater
> could miss an extremely large number of srcu_read_unlock()s. However,
> each missed srcu_read_unlock() must have a corresponding srcu_read_lock(),
> and there is a full memory barrier between between the srcu_flip() and
> the read of the lock count. There is also a full barrier between any
> srcu_read_lock()'s increment of the lock count and that CPU's/task's next
> srcu_read_lock()'s fetch of the index. Therefore, if the updater misses
> counting a given srcu_read_lock(), that CPU's/task's next srcu_read_lock()
> must see the new value of the index. Because srcu_read_lock() disables
> preemption across the index fetch and the lock increment, there can be at
> most NR_CPUS-1 srcu_read_lock() calls that missed the recent srcu_flip()'s
> update to the index. (I said NR_CPUS earlier, but Mathieu is correct
> in pointing out that srcu_flip() has to run somewhere.)
>
> The maximum number of locks that the updater can see is therefore:
>
> o ULONG_MAX/4 for a full set of missed srcu_read_unlock()s.
>
> o ULONG_MAX/4 for a full set of srcu_read_lock()s.
>
> o NR_CPUS-1 for a full set of subsequent srcu_read_lock()s that
> missed the flip.
>
> This totals to ULONG_MAX/2+NR_CPUS-1. So as long as there are no more
> than ULONG_MAX/2 CPUs, we should be good. And given that the biggest
> system I have hard evidence of is 4K CPUs, we have ample headrooom
> compared to the ~2G value of ULONG_MAX/2, even on 32-bit systems.
>
> So, what am I missing here?

Your analysis makes sense.

An alternative approach would be to document some limits on the allowed
nesting count for a given SRCU domain that should be expected from a thread,
and use that as an even lower upper bound on the number of concurrent
SRCU readers, which would allow us to increase the number of supported
CPUs accordingly on 32-bit systems.

Thanks,

Mathieu

>
> Thanx, Paul
>
>> Suggested-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
>> Signed-off-by: Lance Roy <ldr709@xxxxxxxxx>
>> ---
>> include/linux/srcu.h | 4 +-
>> kernel/rcu/rcutorture.c | 20 ++++++++-
>> kernel/rcu/srcu.c | 116 ++++++++++++++++++------------------------------
>> 3 files changed, 63 insertions(+), 77 deletions(-)
>>
>> diff --git a/include/linux/srcu.h b/include/linux/srcu.h
>> index dc8eb63..0caea34 100644
>> --- a/include/linux/srcu.h
>> +++ b/include/linux/srcu.h
>> @@ -34,8 +34,8 @@
>> #include <linux/workqueue.h>
>>
>> struct srcu_struct_array {
>> - unsigned long c[2];
>> - unsigned long seq[2];
>> + unsigned long lock_count[2];
>> + unsigned long unlock_count[2];
>> };
>>
>> struct rcu_batch {
>> diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
>> index bf08fee..2450c61 100644
>> --- a/kernel/rcu/rcutorture.c
>> +++ b/kernel/rcu/rcutorture.c
>> @@ -555,10 +555,26 @@ static void srcu_torture_stats(void)
>> pr_alert("%s%s per-CPU(idx=%d):",
>> torture_type, TORTURE_FLAG, idx);
>> for_each_possible_cpu(cpu) {
>> + unsigned long l0, l1;
>> + unsigned long u0, u1;
>> long c0, c1;
>> + struct srcu_struct_array *counts =
>> + per_cpu_ptr(srcu_ctlp->per_cpu_ref, cpu);
>>
>> - c0 = (long)per_cpu_ptr(srcu_ctlp->per_cpu_ref, cpu)->c[!idx];
>> - c1 = (long)per_cpu_ptr(srcu_ctlp->per_cpu_ref, cpu)->c[idx];
>> + u0 = counts->unlock_count[!idx];
>> + u1 = counts->unlock_count[idx];
>> +
>> + /*
>> + * Make sure that a lock is always counted if the corresponding
>> + * unlock is counted.
>> + */
>> + smp_rmb();
>> +
>> + l0 = counts->lock_count[!idx];
>> + l1 = counts->lock_count[idx];
>> +
>> + c0 = (long)(l0 - u0);
>> + c1 = (long)(l1 - u1);
>> pr_cont(" %d(%ld,%ld)", cpu, c0, c1);
>> }
>> pr_cont("\n");
>> diff --git a/kernel/rcu/srcu.c b/kernel/rcu/srcu.c
>> index 9b9cdd5..38e9aae 100644
>> --- a/kernel/rcu/srcu.c
>> +++ b/kernel/rcu/srcu.c
>> @@ -141,34 +141,38 @@ EXPORT_SYMBOL_GPL(init_srcu_struct);
>> #endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
>>
>> /*
>> - * Returns approximate total of the readers' ->seq[] values for the
>> + * Returns approximate total of the readers' ->lock_count[] values for the
>> * rank of per-CPU counters specified by idx.
>> */
>> -static unsigned long srcu_readers_seq_idx(struct srcu_struct *sp, int idx)
>> +static unsigned long srcu_readers_lock_idx(struct srcu_struct *sp, int idx)
>> {
>> int cpu;
>> unsigned long sum = 0;
>> unsigned long t;
>>
>> for_each_possible_cpu(cpu) {
>> - t = READ_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->seq[idx]);
>> + struct srcu_struct_array *cpu_counts =
>> + per_cpu_ptr(sp->per_cpu_ref, cpu);
>> + t = READ_ONCE(cpu_counts->lock_count[idx]);
>> sum += t;
>> }
>> return sum;
>> }
>>
>> /*
>> - * Returns approximate number of readers active on the specified rank
>> - * of the per-CPU ->c[] counters.
>> + * Returns approximate total of the readers' ->unlock_count[] values for the
>> + * rank of per-CPU counters specified by idx.
>> */
>> -static unsigned long srcu_readers_active_idx(struct srcu_struct *sp, int idx)
>> +static unsigned long srcu_readers_unlock_idx(struct srcu_struct *sp, int idx)
>> {
>> int cpu;
>> unsigned long sum = 0;
>> unsigned long t;
>>
>> for_each_possible_cpu(cpu) {
>> - t = READ_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[idx]);
>> + struct srcu_struct_array *cpu_counts =
>> + per_cpu_ptr(sp->per_cpu_ref, cpu);
>> + t = READ_ONCE(cpu_counts->unlock_count[idx]);
>> sum += t;
>> }
>> return sum;
>> @@ -176,79 +180,42 @@ static unsigned long srcu_readers_active_idx(struct
>> srcu_struct *sp, int idx)
>>
>> /*
>> * Return true if the number of pre-existing readers is determined to
>> - * be stably zero. An example unstable zero can occur if the call
>> - * to srcu_readers_active_idx() misses an __srcu_read_lock() increment,
>> - * but due to task migration, sees the corresponding __srcu_read_unlock()
>> - * decrement. This can happen because srcu_readers_active_idx() takes
>> - * time to sum the array, and might in fact be interrupted or preempted
>> - * partway through the summation.
>> + * be zero.
>> */
>> static bool srcu_readers_active_idx_check(struct srcu_struct *sp, int idx)
>> {
>> - unsigned long seq;
>> + unsigned long unlocks;
>>
>> - seq = srcu_readers_seq_idx(sp, idx);
>> + unlocks = srcu_readers_unlock_idx(sp, idx);
>>
>> /*
>> - * The following smp_mb() A pairs with the smp_mb() B located in
>> - * __srcu_read_lock(). This pairing ensures that if an
>> - * __srcu_read_lock() increments its counter after the summation
>> - * in srcu_readers_active_idx(), then the corresponding SRCU read-side
>> - * critical section will see any changes made prior to the start
>> - * of the current SRCU grace period.
>> + * Make sure that a lock is always counted if the corresponding unlock
>> + * is counted. Needs to be a smp_mb() as the read side may contain a
>> + * read from a variable that is written to before the synchronize_srcu()
>> + * in the write side. In this case smp_mb()s A and B act like the store
>> + * buffering pattern.
>> *
>> - * Also, if the above call to srcu_readers_seq_idx() saw the
>> - * increment of ->seq[], then the call to srcu_readers_active_idx()
>> - * must see the increment of ->c[].
>> + * This smp_mb() also pairs with smp_mb() C to prevent writes after the
>> + * synchronize_srcu() from being executed before the grace period ends.
>> */
>> smp_mb(); /* A */
>>
>> /*
>> - * Note that srcu_readers_active_idx() can incorrectly return
>> - * zero even though there is a pre-existing reader throughout.
>> - * To see this, suppose that task A is in a very long SRCU
>> - * read-side critical section that started on CPU 0, and that
>> - * no other reader exists, so that the sum of the counters
>> - * is equal to one. Then suppose that task B starts executing
>> - * srcu_readers_active_idx(), summing up to CPU 1, and then that
>> - * task C starts reading on CPU 0, so that its increment is not
>> - * summed, but finishes reading on CPU 2, so that its decrement
>> - * -is- summed. Then when task B completes its sum, it will
>> - * incorrectly get zero, despite the fact that task A has been
>> - * in its SRCU read-side critical section the whole time.
>> - *
>> - * We therefore do a validation step should srcu_readers_active_idx()
>> - * return zero.
>> - */
>> - if (srcu_readers_active_idx(sp, idx) != 0)
>> - return false;
>> -
>> - /*
>> - * The remainder of this function is the validation step.
>> - * The following smp_mb() D pairs with the smp_mb() C in
>> - * __srcu_read_unlock(). If the __srcu_read_unlock() was seen
>> - * by srcu_readers_active_idx() above, then any destructive
>> - * operation performed after the grace period will happen after
>> - * the corresponding SRCU read-side critical section.
>> + * If the locks are the same as the unlocks, then there must of have
>> + * been no readers on this index at some time in between. This does not
>> + * mean that there are no more readers, as one could have read the
>> + * current index but have incremented the lock counter yet.
>> *
>> - * Note that there can be at most NR_CPUS worth of readers using
>> - * the old index, which is not enough to overflow even a 32-bit
>> - * integer. (Yes, this does mean that systems having more than
>> - * a billion or so CPUs need to be 64-bit systems.) Therefore,
>> - * the sum of the ->seq[] counters cannot possibly overflow.
>> - * Therefore, the only way that the return values of the two
>> - * calls to srcu_readers_seq_idx() can be equal is if there were
>> - * no increments of the corresponding rank of ->seq[] counts
>> - * in the interim. But the missed-increment scenario laid out
>> - * above includes an increment of the ->seq[] counter by
>> - * the corresponding __srcu_read_lock(). Therefore, if this
>> - * scenario occurs, the return values from the two calls to
>> - * srcu_readers_seq_idx() will differ, and thus the validation
>> - * step below suffices.
>> + * Possible bug: There is no guarantee that there haven't been ULONG_MAX
>> + * increments of ->lock_count[] since the unlocks were counted, meaning
>> + * that this could return true even if there are still active readers.
>> + * Since there are no memory barriers around srcu_flip(), the CPU is not
>> + * required to increment ->completed before running
>> + * srcu_readers_unlock_idx(), which means that there could be an
>> + * arbitrarily large number of critical sections that execute after
>> + * srcu_readers_unlock_idx() but use the old value of ->completed.
>> */
>> - smp_mb(); /* D */
>> -
>> - return srcu_readers_seq_idx(sp, idx) == seq;
>> + return srcu_readers_lock_idx(sp, idx) == unlocks;
>> }
>>
>> /**
>> @@ -266,8 +233,12 @@ static bool srcu_readers_active(struct srcu_struct *sp)
>> unsigned long sum = 0;
>>
>> for_each_possible_cpu(cpu) {
>> - sum += READ_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[0]);
>> - sum += READ_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[1]);
>> + struct srcu_struct_array *cpu_counts =
>> + per_cpu_ptr(sp->per_cpu_ref, cpu);
>> + sum += READ_ONCE(cpu_counts->lock_count[0]);
>> + sum += READ_ONCE(cpu_counts->lock_count[1]);
>> + sum -= READ_ONCE(cpu_counts->unlock_count[0]);
>> + sum -= READ_ONCE(cpu_counts->unlock_count[1]);
>> }
>> return sum;
>> }
>> @@ -298,9 +269,8 @@ int __srcu_read_lock(struct srcu_struct *sp)
>> int idx;
>>
>> idx = READ_ONCE(sp->completed) & 0x1;
>> - __this_cpu_inc(sp->per_cpu_ref->c[idx]);
>> + __this_cpu_inc(sp->per_cpu_ref->lock_count[idx]);
>> smp_mb(); /* B */ /* Avoid leaking the critical section. */
>> - __this_cpu_inc(sp->per_cpu_ref->seq[idx]);
>> return idx;
>> }
>> EXPORT_SYMBOL_GPL(__srcu_read_lock);
>> @@ -314,7 +284,7 @@ EXPORT_SYMBOL_GPL(__srcu_read_lock);
>> void __srcu_read_unlock(struct srcu_struct *sp, int idx)
>> {
>> smp_mb(); /* C */ /* Avoid leaking the critical section. */
>> - this_cpu_dec(sp->per_cpu_ref->c[idx]);
>> + this_cpu_inc(sp->per_cpu_ref->unlock_count[idx]);
>> }
>> EXPORT_SYMBOL_GPL(__srcu_read_unlock);
>>
>> @@ -349,7 +319,7 @@ static bool try_check_zero(struct srcu_struct *sp, int idx,
>> int trycount)
>>
>> /*
>> * Increment the ->completed counter so that future SRCU readers will
>> - * use the other rank of the ->c[] and ->seq[] arrays. This allows
>> + * use the other rank of the ->(un)lock_count[] arrays. This allows
>> * us to wait for pre-existing readers in a starvation-free manner.
>> */
>> static void srcu_flip(struct srcu_struct *sp)
>> --
>> 2.9.0

--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com