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Date: Thu, 22 Dec 2022 10:23:17 -0500
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Michał Cłapiński <mclapinski@...gle.com>
Cc: "Paul E. McKenney" <paulmck@...nel.org>,
Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Andrei Vagin <avagin@...il.com>, Shuah Khan <shuah@...nel.org>,
linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH 1/2] sched/membarrier: Introduce
MEMBARRIER_CMD_GET_REGISTRATIONS
On 2022-12-20 12:51, Michał Cłapiński wrote:
> On Wed, Dec 7, 2022 at 7:04 PM Michał Cłapiński <mclapinski@...gle.com> wrote:
>>
>> On Wed, Dec 7, 2022 at 6:07 PM Mathieu Desnoyers
>> <mathieu.desnoyers@...icios.com> wrote:
>>>
>>> On 2022-12-07 11:43, Michal Clapinski wrote:
>>>> Provide a method to query previously issued registrations.
>>>>
>>>> Signed-off-by: Michal Clapinski <mclapinski@...gle.com>
>>>> ---
>>>> include/uapi/linux/membarrier.h | 4 ++++
>>>> kernel/sched/membarrier.c | 39 ++++++++++++++++++++++++++++++++-
>>>> 2 files changed, 42 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/include/uapi/linux/membarrier.h b/include/uapi/linux/membarrier.h
>>>> index 737605897f36..5f3ad6d5be6f 100644
>>>> --- a/include/uapi/linux/membarrier.h
>>>> +++ b/include/uapi/linux/membarrier.h
>>>> @@ -137,6 +137,9 @@
>>>> * @MEMBARRIER_CMD_SHARED:
>>>> * Alias to MEMBARRIER_CMD_GLOBAL. Provided for
>>>> * header backward compatibility.
>>>> + * @MEMBARRIER_CMD_GET_REGISTRATIONS:
>>>> + * Returns a bitmask of previously issued
>>>> + * registration commands.
>>>> *
>>>> * Command to be passed to the membarrier system call. The commands need to
>>>> * be a single bit each, except for MEMBARRIER_CMD_QUERY which is assigned to
>>>> @@ -153,6 +156,7 @@ enum membarrier_cmd {
>>>> MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_SYNC_CORE = (1 << 6),
>>>> MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ = (1 << 7),
>>>> MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_RSEQ = (1 << 8),
>>>> + MEMBARRIER_CMD_GET_REGISTRATIONS = (1 << 9),
>>
>> Btw. I could do this as a flag to MEMBARRIER_CMD_QUERY instead of a
>> separate command. Would that be preferable?
I do not think that would be better, no. We can keep it with
GET_REGISTRATIONS.
>>
>>
>>>>
>>>> /* Alias for header backward compatibility. */
>>>> MEMBARRIER_CMD_SHARED = MEMBARRIER_CMD_GLOBAL,
>>>> diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c
>>>> index 0c5be7ebb1dc..2ad881d07752 100644
>>>> --- a/kernel/sched/membarrier.c
>>>> +++ b/kernel/sched/membarrier.c
>>>> @@ -159,7 +159,8 @@
>>>> | MEMBARRIER_CMD_PRIVATE_EXPEDITED \
>>>> | MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED \
>>>> | MEMBARRIER_PRIVATE_EXPEDITED_SYNC_CORE_BITMASK \
>>>> - | MEMBARRIER_PRIVATE_EXPEDITED_RSEQ_BITMASK)
>>>> + | MEMBARRIER_PRIVATE_EXPEDITED_RSEQ_BITMASK \
>>>> + | MEMBARRIER_CMD_GET_REGISTRATIONS)
>>>>
>>>> static void ipi_mb(void *info)
>>>> {
>>>> @@ -540,6 +541,40 @@ static int membarrier_register_private_expedited(int flags)
>>>> return 0;
>>>> }
>>>>
>>>> +static int membarrier_get_registrations(void)
>>>> +{
>>>> + struct task_struct *p = current;
>>>> + struct mm_struct *mm = p->mm;
>>>> + int registrations_mask = 0, membarrier_state, i;
>>>> + static const int states[] = {
>>>> + MEMBARRIER_STATE_GLOBAL_EXPEDITED |
>>>> + MEMBARRIER_STATE_GLOBAL_EXPEDITED_READY,
>>>
>>> What is the purpose of checking for the _READY state flag as well here ?
>>
>> Answered below.
>>
>>
>>>
>>>
>>>> + MEMBARRIER_STATE_PRIVATE_EXPEDITED |
>>>> + MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY,
>>>> + MEMBARRIER_STATE_PRIVATE_EXPEDITED_SYNC_CORE |
>>>> + MEMBARRIER_STATE_PRIVATE_EXPEDITED_SYNC_CORE_READY,
>>>> + MEMBARRIER_STATE_PRIVATE_EXPEDITED_RSEQ |
>>>> + MEMBARRIER_STATE_PRIVATE_EXPEDITED_RSEQ_READY
>>>> + };
>>>> + static const int registration_cmds[] = {
>>>> + MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED,
>>>> + MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED,
>>>> + MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_SYNC_CORE,
>>>> + MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_RSEQ
>>>> + };
>>>> + BUILD_BUG_ON(ARRAY_SIZE(states) != ARRAY_SIZE(registration_cmds));
>>>> +
>>>> + membarrier_state = atomic_read(&mm->membarrier_state);
>>>> + for (i = 0; i < ARRAY_SIZE(states); ++i) {
>>>> + if (membarrier_state & states[i]) {
>>>
>>> The mask will match if either of the flags to match are set. Is that
>>> your intent ?
>>
>> Kind of, it was just the easiest to write. As explained in the cover
>> letter, I don't really care much about the result of this while the
>> process is running. And when the process is frozen, either state and
>> state_ready are set or none of them.
OK
>>
>>
>>>
>>>
>>>> + registrations_mask |= registration_cmds[i];
>>>> + membarrier_state &= ~states[i];
>>>
>>> So I understand that those _READY flags are there purely for making sure
>>> we clear the membarrier_state for validation that they have all been
>>> checked with the following WARN_ON_ONCE(). Am I on the right track ?
>>
>> Yes, exactly. It wastes time but I'm worried about people adding new
>> states and not updating this function. A suggestion on how to do this
>> better (especially at compile time) would be greatly appreciated.
Although it's not a fast-path, so let's keep it this way for now.
>>
>>
>>>
>>>> + }
>>>> + }
>>>> + WARN_ON_ONCE(membarrier_state != 0);
>>>
>>> Thanks,
>>>
>>> Mathieu
>>>
>>>> + return registrations_mask;
>>>> +}
>>>> +
>>>> /**
>>>> * sys_membarrier - issue memory barriers on a set of threads
>>>> * @cmd: Takes command values defined in enum membarrier_cmd.
>>>> @@ -623,6 +658,8 @@ SYSCALL_DEFINE3(membarrier, int, cmd, unsigned int, flags, int, cpu_id)
>>>> return membarrier_private_expedited(MEMBARRIER_FLAG_RSEQ, cpu_id);
>>>> case MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_RSEQ:
>>>> return membarrier_register_private_expedited(MEMBARRIER_FLAG_RSEQ);
>>>> + case MEMBARRIER_CMD_GET_REGISTRATIONS:
>>>> + return membarrier_get_registrations();
>>>> default:
>>>> return -EINVAL;
>>>> }
>>>
>>> --
>>> Mathieu Desnoyers
>>> EfficiOS Inc.
>>> https://www.efficios.com
>>>
>
> Hi Mathieu,
> is there anything more you need from my side?
No, I think those patches are ok.
Thanks,
Mathieu
--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com
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