Re: [PATCH v9 10/15] x86/sgx: Add EPC reclamation in cgroup try_charge()

["Haitao Huang" <haitao.huang@linux.intel.com>]

Hi Jarkko

On Mon, 12 Feb 2024 13:55:46 -0600, Jarkko Sakkinen <jarkko@kernel.org>  
wrote:

> On Mon Feb 5, 2024 at 11:06 PM EET, Haitao Huang wrote:
>> From: Kristen Carlson Accardi <kristen@linux.intel.com>
>>
>> When the EPC usage of a cgroup is near its limit, the cgroup needs to
>> reclaim pages used in the same cgroup to make room for new allocations.
>> This is analogous to the behavior that the global reclaimer is triggered
>> when the global usage is close to total available EPC.
>>
>> Add a Boolean parameter for sgx_epc_cgroup_try_charge() to indicate
>> whether synchronous reclaim is allowed or not. And trigger the
>> synchronous/asynchronous reclamation flow accordingly.
>>
>> Note at this point, all reclaimable EPC pages are still tracked in the
>> global LRU and per-cgroup LRUs are empty. So no per-cgroup reclamation
>> is activated yet.
>>
>> Co-developed-by: Sean Christopherson <sean.j.christopherson@intel.com>
>> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
>> Signed-off-by: Kristen Carlson Accardi <kristen@linux.intel.com>
>> Co-developed-by: Haitao Huang <haitao.huang@linux.intel.com>
>> Signed-off-by: Haitao Huang <haitao.huang@linux.intel.com>
>> ---
>> V7:
>> - Split this out from the big patch, #10 in V6. (Dave, Kai)
>> ---
>>  arch/x86/kernel/cpu/sgx/epc_cgroup.c | 26 ++++++++++++++++++++++++--
>>  arch/x86/kernel/cpu/sgx/epc_cgroup.h |  4 ++--
>>  arch/x86/kernel/cpu/sgx/main.c       |  2 +-
>>  3 files changed, 27 insertions(+), 5 deletions(-)
>>
>> diff --git a/arch/x86/kernel/cpu/sgx/epc_cgroup.c  
>> b/arch/x86/kernel/cpu/sgx/epc_cgroup.c
>> index d399fda2b55e..abf74fdb12b4 100644
>> --- a/arch/x86/kernel/cpu/sgx/epc_cgroup.c
>> +++ b/arch/x86/kernel/cpu/sgx/epc_cgroup.c
>> @@ -184,13 +184,35 @@ static void  
>> sgx_epc_cgroup_reclaim_work_func(struct work_struct *work)
>>  /**
>>   * 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.
>> + * @reclaim:	Whether or not synchronous reclaim is allowed
>>   * Return:
>>   * * %0 - If successfully charged.
>>   * * -errno - for failures.
>>   */
>> -int sgx_epc_cgroup_try_charge(struct sgx_epc_cgroup *epc_cg)
>> +int sgx_epc_cgroup_try_charge(struct sgx_epc_cgroup *epc_cg, bool  
>> reclaim)
>>  {
>> -	return misc_cg_try_charge(MISC_CG_RES_SGX_EPC, epc_cg->cg, PAGE_SIZE);
>> +	for (;;) {
>> +		if (!misc_cg_try_charge(MISC_CG_RES_SGX_EPC, epc_cg->cg,
>> +					PAGE_SIZE))
>> +			break;
>> +
>> +		if (sgx_epc_cgroup_lru_empty(epc_cg->cg))
>> +			return -ENOMEM;
>> + +		if (signal_pending(current))
>> +			return -ERESTARTSYS;
>> +
>> +		if (!reclaim) {
>> +			queue_work(sgx_epc_cg_wq, &epc_cg->reclaim_work);
>> +			return -EBUSY;
>> +		}
>> +
>> +		if (!sgx_epc_cgroup_reclaim_pages(epc_cg->cg, false))
>> +			/* All pages were too young to reclaim, try again a little later */
>> +			schedule();
>
> This will be total pain to backtrack after a while when something
> needs to be changed so there definitely should be inline comments
> addressing each branch condition.
>
> I'd rethink this as:
>
> 1. Create static __sgx_epc_cgroup_try_charge() for addressing single
>    iteration with the new "reclaim" parameter.
> 2. Add a new sgx_epc_group_try_charge_reclaim() function.
>
> There's a bit of redundancy with sgx_epc_cgroup_try_charge() and
> sgx_epc_cgroup_try_charge_reclaim() because both have almost the
> same loop calling internal __sgx_epc_cgroup_try_charge() with
> different parameters. That is totally acceptable.
>
> Please also add my suggested-by.
>
> BR, Jarkko
>
> BR, Jarkko
>
For #2:
The only caller of this function, sgx_alloc_epc_page(), has the same  
boolean which is passed into this this function.

If we separate it into sgx_epc_cgroup_try_charge() and  
sgx_epc_cgroup_try_charge_reclaim(), then the caller has to have the  
if/else branches. So separation here seems not help?


For #1:
If we don't do #2, It seems overkill at the moment for such a short  
function.

How about we add inline comments for each branch for now, and if later  
there are more branches and the function become too long we add  
__sgx_epc_cgroup_try_charge() as you suggested?

Thanks
Haitao