| Re: [ckrm-tech] [RFC][PATCH][2/4] Add RSS accounting and control [message #10441] |
Mon, 19 February 2007 11:09  |
Balbir Singh
Messages: 491
Registered: August 2006
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Senior Member |
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Andrew Morton wrote:
> On Mon, 19 Feb 2007 16:07:44 +0530 Balbir Singh <balbir@in.ibm.com> wrote:
>
>>>> +void memctlr_mm_free(struct mm_struct *mm)
>>>> +{
>>>> + kfree(mm->counter);
>>>> +}
>>>> +
>>>> +static inline void memctlr_mm_assign_container_direct(struct mm_struct *mm,
>>>> + struct container *cont)
>>>> +{
>>>> + write_lock(&mm->container_lock);
>>>> + mm->container = cont;
>>>> + write_unlock(&mm->container_lock);
>>>> +}
>>> More weird locking here.
>>>
>> The container field of the mm_struct is protected by a read write spin lock.
>
> That doesn't mean anything to me.
>
> What would go wrong if the above locking was simply removed? And how does
> the locking prevent that fault?
>
Some pages could charged to the wrong container. Apart from that I do not
see anything going bad (I'll double check that).
>
>>>> +void memctlr_mm_assign_container(struct mm_struct *mm, struct task_struct *p)
>>>> +{
>>>> + struct container *cont = task_container(p, &memctlr_subsys);
>>>> + struct memctlr *mem = memctlr_from_cont(cont);
>>>> +
>>>> + BUG_ON(!mem);
>>>> + write_lock(&mm->container_lock);
>>>> + mm->container = cont;
>>>> + write_unlock(&mm->container_lock);
>>>> +}
>>> And here.
>> Ditto.
>
> ditto ;)
>
:-)
>>>> +/*
>>>> + * Update the rss usage counters for the mm_struct and the container it belongs
>>>> + * to. We do not fail rss for pages shared during fork (see copy_one_pte()).
>>>> + */
>>>> +int memctlr_update_rss(struct mm_struct *mm, int count, bool check)
>>>> +{
>>>> + int ret = 1;
>>>> + struct container *cont;
>>>> + long usage, limit;
>>>> + struct memctlr *mem;
>>>> +
>>>> + read_lock(&mm->container_lock);
>>>> + cont = mm->container;
>>>> + read_unlock(&mm->container_lock);
>>>> +
>>>> + if (!cont)
>>>> + goto done;
>>> And here. I mean, if there was a reason for taking the lock around that
>>> read, then testing `cont' outside the lock just invalidated that reason.
>>>
>> We took a consistent snapshot of cont. It cannot change outside the lock,
>> we check the value outside. I am sure I missed something.
>
> If it cannot change outside the lock then we don't need to take the lock!
>
We took a snapshot that we thought was consistent. We check for the value
outside. I guess there is no harm, the worst thing that could happen
is wrong accounting during mm->container changes (when a task changes
container).
>> MEMCTLR_DONT_CHECK_LIMIT exists for the following reasons
>>
>> 1. Pages are shared during fork, fork() is not failed at that point
>> since the pages are shared anyway, we allow the RSS limit to be
>> exceeded.
>> 2. When ZERO_PAGE is added, we don't check for limits (zeromap_pte_range).
>> 3. On reducing RSS (passing -1 as the value)
>
> OK, that might make a nice comment somewhere (if it's not already there).
Yes, thanks for keeping us humble and honest, I'll add it.
--
Warm Regards,
Balbir Singh
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