Date: Thu, 22 Feb 2024 12:09:28 -0600
From: "Haitao Huang" <haitao.huang@...ux.intel.com>
To: "hpa@...or.com" <hpa@...or.com>, "tim.c.chen@...ux.intel.com"
 <tim.c.chen@...ux.intel.com>, "linux-sgx@...r.kernel.org"
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 <yangjie@...rosoft.com>, "Li, Zhiquan1" <zhiquan1.li@...el.com>,
 "chrisyan@...rosoft.com" <chrisyan@...rosoft.com>
Subject: Re: [PATCH v9 08/15] x86/sgx: Implement EPC reclamation flows for
 cgroup

On Tue, 20 Feb 2024 03:52:39 -0600, Huang, Kai <kai.huang@...el.com> wrote:

>>> +/**
>> + * sgx_epc_cgroup_reclaim_pages() - walk a cgroup tree and scan LRUs  
>> to reclaim pages
>> + * @root:	Root of the tree to start walking from.
>> + * Return:	Number of pages reclaimed.
>
> Just wondering, do you need to return @cnt given this function is called  
> w/o
> checking the return value?
>
Yes. Will add explicit commenting that we need scan fixed number of pages  
for attempted reclamation.
>> + */
>> +unsigned int sgx_epc_cgroup_reclaim_pages(struct misc_cg *root)
>> +{
>> +	/*
>> +	 * Attempting to reclaim only a few pages will often fail and is
>> +	 * inefficient, while reclaiming a huge number of pages can result in
>> +	 * soft lockups due to holding various locks for an extended duration.
>> +	 */
>
> Not sure we need this comment, given it's already implied in
> sgx_reclaim_pages().  You cannot pass a value > SGX_NR_TO_SCAN anyway.

Will rework on these comments to make them more meaningful.
>

[other comments/questions addressed in separate email threads]
[...]
>> +
>> +/*
>> + * Scheduled by sgx_epc_cgroup_try_charge() to reclaim pages from the  
>> cgroup
>> + * when the cgroup is at/near its maximum capacity
>> + */
>
> I don't see this being "scheduled by sgx_epc_cgroup_try_charge()" here.   
> Does it
> make more sense to move that code change to this patch for better review?
>

Right. This comment was left-over when I split the old patch.

>> +static void sgx_epc_cgroup_reclaim_work_func(struct work_struct *work)
>> +{
>> +	struct sgx_epc_cgroup *epc_cg;
>> +	u64 cur, max;
>> +
>> +	epc_cg = container_of(work, struct sgx_epc_cgroup, reclaim_work);
>> +
>> +	for (;;) {
>> +		max = sgx_epc_cgroup_max_pages_to_root(epc_cg);
>> +
>> +		/*
>> +		 * Adjust the limit down by one page, the goal is to free up
>> +		 * pages for fault allocations, not to simply obey the limit.
>> +		 * Conditionally decrementing max also means the cur vs. max
>> +		 * check will correctly handle the case where both are zero.
>> +		 */
>> +		if (max)
>> +			max--;
>
> With the below max -= SGX_NR_TO_SCAN/2 staff, do you still need this one?
>

Logically still needed for case max <= SGX_NR_TO_SCAN * 2

>> +
>> +		/*
>> +		 * Unless the limit is extremely low, in which case forcing
>> +		 * reclaim will likely cause thrashing, force the cgroup to
>> +		 * reclaim at least once if it's operating *near* its maximum
>> +		 * limit by adjusting @max down by half the min reclaim size.
>
> OK.  But why choose "SGX_NO_TO_SCAN * 2" as "extremely low"? E.g, could  
> we
> choose SGX_NR_TO_SCAN instead?
> IMHO at least we should at least put a comment to mention this.
>
> And maybe you can have a dedicated macro for that in which way I believe  
> the
> code would be easier to understand?

Good point. I think the value is kind of arbitrary. We consider  
enclaves/cgroups of 64K size are very small. If such a cgroup ever reaches  
the limit, then we don't aggressively reclaim to optimize #PF handling.  
User might as well just raise the limit if it is not performant.

>
>> +		 * This work func is scheduled by sgx_epc_cgroup_try_charge
>
> This has been mentioned in the function comment already.
>
>> +		 * when it cannot directly reclaim due to being in an atomic
>> +		 * context, e.g. EPC allocation in a fault handler.
>
> Why a fault handler is an "atomic context"?  Just say when it cannot  
> directly
> reclaim.
>

Sure.

>> Waiting
>> +		 * to reclaim until the cgroup is actually at its limit is less
>> +		 * performant as it means the faulting task is effectively
>> +		 * blocked until a worker makes its way through the global work
>> +		 * queue.
>> +		 */
>> +		if (max > SGX_NR_TO_SCAN * 2)
>> +			max -= (SGX_NR_TO_SCAN / 2);
>> +
>> +		cur = sgx_epc_cgroup_page_counter_read(epc_cg);
>> +
>> +		if (cur <= max || sgx_epc_cgroup_lru_empty(epc_cg->cg))
>> +			break;
>> +
>> +		/* Keep reclaiming until above condition is met. */
>> +		sgx_epc_cgroup_reclaim_pages(epc_cg->cg);
>
> Also, each loop here calls sgx_epc_cgroup_max_pages_to_root() and
> sgx_epc_cgroup_lru_empty(), both loop the given EPC cgroup and  
> descendants.  If
> we still make sgx_reclaim_pages() always scan SGX_NR_TO_SCAN pages,  
> seems we can
> reduce the number of loops here?
>

[We already scan SGX_NR_TO_SCAN pages for the cgroup at the level of  
sgx_epc_cgroup_reclaim_pages().]

I think you mean that we keep scanning and reclaiming until at least  
SGX_NR_TO_SCAN pages are reclaimed as your code suggested above. We  
probably can make that a version for this background thread for  
optimization. But sgx_epc_cgroup_max_pages_to_root() and  
sgx_epc_cgroup_lru_empty() are not that bad unless we had very deep and  
wide cgroup trees. So would you agree we defer this optimization for later?


>> +	}
>> +}
>> +
>> +/**
>> + * sgx_epc_cgroup_try_charge() - try to charge cgroup for a single EPC  
>> page
>>   * @epc_cg:	The EPC cgroup to be charged for the page.
>>   * Return:
>>   * * %0 - If successfully charged.
>> @@ -38,6 +209,7 @@ static void sgx_epc_cgroup_free(struct misc_cg *cg)
>>  	if (!epc_cg)
>>  		return;
>>
>> +	cancel_work_sync(&epc_cg->reclaim_work);
>>  	kfree(epc_cg);
>>  }
>>
>> @@ -50,6 +222,8 @@ const struct misc_res_ops sgx_epc_cgroup_ops = {
>>
>>  static void sgx_epc_misc_init(struct misc_cg *cg, struct  
>> sgx_epc_cgroup *epc_cg)
>>  {
>> +	sgx_lru_init(&epc_cg->lru);
>> +	INIT_WORK(&epc_cg->reclaim_work, sgx_epc_cgroup_reclaim_work_func);
>>  	cg->res[MISC_CG_RES_SGX_EPC].priv = epc_cg;
>>  	epc_cg->cg = cg;
>>  }
>> @@ -69,6 +243,11 @@ static int sgx_epc_cgroup_alloc(struct misc_cg *cg)
>>
>>  void sgx_epc_cgroup_init(void)
>>  {
>> +	sgx_epc_cg_wq = alloc_workqueue("sgx_epc_cg_wq",
>> +					WQ_UNBOUND | WQ_FREEZABLE,
>> +					WQ_UNBOUND_MAX_ACTIVE);
>> +	BUG_ON(!sgx_epc_cg_wq);
>
> You cannot BUG_ON() simply due to unable to allocate a workqueue.  You  
> can use
> some way to mark EPC cgroup as disabled but keep going.  Static key is  
> one way
> although we cannot re-enable it at runtime.
>
>
Okay, I'll disable and print a log.

[...]
[workqueue related discussion in separate email]

Thanks
Haitao

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