Date:   Wed, 5 Jul 2023 11:27:28 +0800
From:   Qi Zheng <zhengqi.arch@...edance.com>
To:     paulmck@...nel.org, Dave Chinner <david@...morbit.com>
Cc:     Vlastimil Babka <vbabka@...e.cz>, akpm@...ux-foundation.org,
        tkhai@...ru, roman.gushchin@...ux.dev, djwong@...nel.org,
        brauner@...nel.org, tytso@....edu, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, intel-gfx@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org, linux-arm-msm@...r.kernel.org,
        dm-devel@...hat.com, linux-raid@...r.kernel.org,
        linux-bcache@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-nfs@...r.kernel.org, linux-xfs@...r.kernel.org,
        linux-btrfs@...r.kernel.org
Subject: Re: [PATCH 24/29] mm: vmscan: make global slab shrink lockless



On 2023/7/4 11:45, Qi Zheng wrote:
> 
> 
> On 2023/7/4 00:39, Paul E. McKenney wrote:
>> On Fri, Jun 23, 2023 at 04:29:39PM +1000, Dave Chinner wrote:
>>> On Thu, Jun 22, 2023 at 05:12:02PM +0200, Vlastimil Babka wrote:
>>>> On 6/22/23 10:53, Qi Zheng wrote:
>>>>> @@ -1067,33 +1068,27 @@ static unsigned long shrink_slab(gfp_t 
>>>>> gfp_mask, int nid,
>>>>>       if (!mem_cgroup_disabled() && !mem_cgroup_is_root(memcg))
>>>>>           return shrink_slab_memcg(gfp_mask, nid, memcg, priority);
>>>>> -    if (!down_read_trylock(&shrinker_rwsem))
>>>>> -        goto out;
>>>>> -
>>>>> -    list_for_each_entry(shrinker, &shrinker_list, list) {
>>>>> +    rcu_read_lock();
>>>>> +    list_for_each_entry_rcu(shrinker, &shrinker_list, list) {
>>>>>           struct shrink_control sc = {
>>>>>               .gfp_mask = gfp_mask,
>>>>>               .nid = nid,
>>>>>               .memcg = memcg,
>>>>>           };
>>>>> +        if (!shrinker_try_get(shrinker))
>>>>> +            continue;
>>>>> +        rcu_read_unlock();
>>>>
>>>> I don't think you can do this unlock?
>>
>> Sorry to be slow to respond here, this one fell through the cracks.
>> And thank you to Qi for reminding me!
>>
>> If you do this unlock, you had jolly well better nail down the current
>> element (the one referenced by shrinker), for example, by acquiring an
>> explicit reference count on the object.  And presumably this is exactly
>> what shrinker_try_get() is doing.  And a look at your 24/29 confirms 
>> this,
>> at least assuming that shrinker->refcount is set to zero before the call
>> to synchronize_rcu() in free_module() *and* that synchronize_rcu() 
>> doesn't
>> start until *after* shrinker_put() calls complete().  Plus, as always,
>> the object must be removed from the list before the synchronize_rcu()
>> starts.  (On these parts of the puzzle, I defer to those more familiar
>> with this code path.  And I strongly suggest carefully commenting this
>> type of action-at-a-distance design pattern.)
> 
> Yeah, I think I've done it like above. A more detailed timing diagram is
> below.
> 
>>
>> Why is this important?  Because otherwise that object might be freed
>> before you get to the call to rcu_read_lock() at the end of this loop.
>> And if that happens, list_for_each_entry_rcu() will be walking the
>> freelist, which is quite bad for the health and well-being of your 
>> kernel.
>>
>> There are a few other ways to make this sort of thing work:
>>
>> 1.    Defer the shrinker_put() to the beginning of the loop.
>>     You would need a flag initially set to zero, and then set to
>>     one just before (or just after) the rcu_read_lock() above.
>>     You would also need another shrinker_old pointer to track the
>>     old pointer.  Then at the top of the loop, if the flag is set,
>>     invoke shrinker_put() on shrinker_old.    This ensures that the
>>     previous shrinker structure stays around long enough to allow
>>     the loop to find the next shrinker structure in the list.
>>
>>     This approach is attractive when the removal code path
>>     can invoke shrinker_put() after the grace period ends.
>>
>> 2.    Make shrinker_put() invoke call_rcu() when ->refcount reaches
>>     zero, and have the callback function free the object.  This of
>>     course requires adding an rcu_head structure to the shrinker
>>     structure, which might or might not be a reasonable course of
>>     action.  If adding that rcu_head is reasonable, this simplifies
>>     the logic quite a bit.
>>
>> 3.    For the shrinker-structure-removal code path, remove the shrinker
>>     structure, then remove the initial count from ->refcount,
>>     and then keep doing grace periods until ->refcount is zero,
>>     then do one more.  Of course, if the result of removing the
>>     initial count was zero, then only a single additional grace
>>     period is required.
>>
>>     This would need to be carefully commented, as it is a bit
>>     unconventional.
> 
> Thanks for such a detailed addition!
> 
>>
>> There are probably many other ways, but just to give an idea of a few
>> other ways to do this.
>>
>>>>> +
>>>>>           ret = do_shrink_slab(&sc, shrinker, priority);
>>>>>           if (ret == SHRINK_EMPTY)
>>>>>               ret = 0;
>>>>>           freed += ret;
>>>>> -        /*
>>>>> -         * Bail out if someone want to register a new shrinker to
>>>>> -         * prevent the registration from being stalled for long 
>>>>> periods
>>>>> -         * by parallel ongoing shrinking.
>>>>> -         */
>>>>> -        if (rwsem_is_contended(&shrinker_rwsem)) {
>>>>> -            freed = freed ? : 1;
>>>>> -            break;
>>>>> -        }
>>>>> -    }
>>>>> -    up_read(&shrinker_rwsem);
>>>>> -out:
>>>>> +        rcu_read_lock();
>>>>
>>>> That new rcu_read_lock() won't help AFAIK, the whole
>>>> list_for_each_entry_rcu() needs to be under the single 
>>>> rcu_read_lock() to be
>>>> safe.
>>>
>>> Yeah, that's the pattern we've been taught and the one we can look
>>> at and immediately say "this is safe".
>>>
>>> This is a different pattern, as has been explained bi Qi, and I
>>> think it *might* be safe.
>>>
>>> *However.*
>>>
>>> Right now I don't have time to go through a novel RCU list iteration
>>> pattern it one step at to determine the correctness of the
>>> algorithm. I'm mostly worried about list manipulations that can
>>> occur outside rcu_read_lock() section bleeding into the RCU
>>> critical section because rcu_read_lock() by itself is not a memory
>>> barrier.
>>>
>>> Maybe Paul has seen this pattern often enough he could simply tell
>>> us what conditions it is safe in. But for me to work that out from
>>> first principles? I just don't have the time to do that right now.
>>
>> If the code does just the right sequence of things on the removal path
>> (remove, decrement reference, wait for reference to go to zero, wait for
>> grace period, free), then it would work.  If this is what is happening,
>> I would argue for more comments.  ;-)
> 
> The order of the removal path is slightly different from this:
> 
>      shrink_slab                 unregister_shrinker
>      ===========                 ===================
> 
>     shrinker_try_get()
>     rcu_read_unlock()
>                                  1. decrement initial reference
>                  shrinker_put()
>                  2. wait for reference to go to zero
>                  wait_for_completion()
>     rcu_read_lock()
> 
>     shrinker_put()
>                  3. remove the shrinker from list
>                  list_del_rcu()
>                                  4. wait for grace period
>                  kfree_rcu()/synchronize_rcu()
> 
> 
>     list_for_each_entry()
> 
>     shrinker_try_get()
>     rcu_read_unlock()
>                  5. free the shrinker
> 
> So the order is: decrement reference, wait for reference to go to zero,
> remove, wait for grace period, free.
> 
> I think this can work. And we can only do the *step 3* after we hold the
> RCU read lock again, right? Please let me know if I missed something.

Oh, you are right, It would be better to move step 3 to step 1. We
should first remove the shrinker from the shrinker_list to prevent
other traversers from finding it again, otherwise the following
situations may occur theoretically:

CPU 0                 CPU 1

shrinker_try_get()

                       shrinker_try_get()

shrinker_put()
shrinker_try_get()
                       shrinker_put()

Thanks,
Qi

> 
> Thanks,
> Qi
> 
>>
>>                             Thanx, Paul
>>
>>>> IIUC this is why Dave in [4] suggests unifying shrink_slab() with
>>>> shrink_slab_memcg(), as the latter doesn't iterate the list but uses 
>>>> IDR.
>>>
>>> Yes, I suggested the IDR route because radix tree lookups under RCU
>>> with reference counted objects are a known safe pattern that we can
>>> easily confirm is correct or not.  Hence I suggested the unification
>>> + IDR route because it makes the life of reviewers so, so much
>>> easier...
>>>
>>> Cheers,
>>>
>>> Dave.
>>> -- 
>>> Dave Chinner
>>> david@...morbit.com

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