From: Jan Beulich <jbeulich@suse.com>
To: "Jürgen Groß" <jgross@suse.com>, "Julien Grall" <julien@xen.org>
Cc: Stefano Stabellini <sstabellini@kernel.org>, Wei Liu <wl@xen.org>,
Andrew Cooper <andrew.cooper3@citrix.com>,
Ian Jackson <ian.jackson@eu.citrix.com>,
George Dunlap <george.dunlap@citrix.com>,
xen-devel@lists.xenproject.org
Subject: Re: [Xen-devel] [PATCH v7 2/5] xen/rcu: don't use stop_machine_run() for rcu_barrier()
Date: Thu, 26 Mar 2020 09:49:37 +0100 [thread overview]
Message-ID: <c060ecc2-3678-2882-8eef-442c2b8c14b9@suse.com> (raw)
In-Reply-To: <7539a91f-6b9a-865d-f8c3-b8124d16f17a@suse.com>
On 26.03.2020 08:24, Jürgen Groß wrote:
> On 26.03.20 07:58, Jan Beulich wrote:
>> On 25.03.2020 17:13, Julien Grall wrote:
>>> On 25/03/2020 10:55, Juergen Gross wrote:
>>>> @@ -143,51 +143,90 @@ static int qhimark = 10000;
>>>> static int qlowmark = 100;
>>>> static int rsinterval = 1000;
>>>> -struct rcu_barrier_data {
>>>> - struct rcu_head head;
>>>> - atomic_t *cpu_count;
>>>> -};
>>>> +/*
>>>> + * rcu_barrier() handling:
>>>> + * Two counters are used to synchronize rcu_barrier() work:
>>>> + * - cpu_count holds the number of cpus required to finish barrier handling.
>>>> + * It is decremented by each cpu when it has performed all pending rcu calls.
>>>> + * - pending_count shows whether any rcu_barrier() activity is running and
>>>> + * it is used to synchronize leaving rcu_barrier() only after all cpus
>>>> + * have finished their processing. pending_count is initialized to nr_cpus + 1
>>>> + * and it is decremented by each cpu when it has seen that cpu_count has
>>>> + * reached 0. The cpu where rcu_barrier() has been called will wait until
>>>> + * pending_count has been decremented to 1 (so all cpus have seen cpu_count
>>>> + * reaching 0) and will then set pending_count to 0 indicating there is no
>>>> + * rcu_barrier() running.
>>>> + * Cpus are synchronized via softirq mechanism. rcu_barrier() is regarded to
>>>> + * be active if pending_count is not zero. In case rcu_barrier() is called on
>>>> + * multiple cpus it is enough to check for pending_count being not zero on entry
>>>> + * and to call process_pending_softirqs() in a loop until pending_count drops to
>>>> + * zero, before starting the new rcu_barrier() processing.
>>>> + */
>>>> +static atomic_t cpu_count = ATOMIC_INIT(0);
>>>> +static atomic_t pending_count = ATOMIC_INIT(0);
>>>> static void rcu_barrier_callback(struct rcu_head *head)
>>>> {
>>>> - struct rcu_barrier_data *data = container_of(
>>>> - head, struct rcu_barrier_data, head);
>>>> - atomic_inc(data->cpu_count);
>>>> + smp_mb__before_atomic(); /* Make all writes visible to other cpus. */
>>>
>>> smp_mb__before_atomic() will order both read and write. However, the
>>> comment suggest only the write are required to be ordered.
>>>
>>> So either the barrier is too strong or the comment is incorrect. Can
>>> you clarify it?
>>
>> Neither is the case, I guess: There simply is no smp_wmb__before_atomic()
>> in Linux, and if we want to follow their model we shouldn't have one
>> either. I'd rather take the comment to indicate that if one appeared, it
>> could be used here.
>
> Right. Currently we have the choice of either using
> smp_mb__before_atomic() which is too strong for Arm, or smp_wmb() which
> is too strong for x86.
For x86 smp_wmb() is actually only very slightly too strong - it expands
to just barrier(), after all. So overall perhaps that's the better
choice here (with a suitable comment)?
Jan
next prev parent reply other threads:[~2020-03-26 8:50 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-25 10:55 [Xen-devel] [PATCH v7 0/5] xen/rcu: let rcu work better with core scheduling Juergen Gross
2020-03-25 10:55 ` [Xen-devel] [PATCH v7 1/5] xen: introduce smp_mb__[after|before]_atomic() barriers Juergen Gross
2020-03-25 13:17 ` Jan Beulich
2020-03-25 16:20 ` Julien Grall
2020-03-25 10:55 ` [Xen-devel] [PATCH v7 2/5] xen/rcu: don't use stop_machine_run() for rcu_barrier() Juergen Gross
2020-03-25 13:19 ` Jan Beulich
2020-03-25 16:13 ` Julien Grall
2020-03-26 6:58 ` Jan Beulich
2020-03-26 7:24 ` Jürgen Groß
2020-03-26 8:49 ` Jan Beulich [this message]
2020-03-26 8:50 ` Jürgen Groß
2020-03-26 9:14 ` Julien Grall
2020-03-25 10:55 ` [Xen-devel] [PATCH v7 3/5] xen: don't process rcu callbacks when holding a rcu_read_lock() Juergen Gross
2020-03-25 10:55 ` [Xen-devel] [PATCH v7 4/5] xen/rcu: add assertions to debug build Juergen Gross
2020-03-25 10:55 ` [Xen-devel] [PATCH v7 5/5] xen/rcu: add per-lock counter in debug builds Juergen Gross
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