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Message-ID: <f242cfb2-e8ab-49f3-9b8e-1236dd361b64@intel.com>
Date: Fri, 23 Feb 2024 11:31:18 +1300
From: "Huang, Kai" <kai.huang@...el.com>
To: Haitao Huang <haitao.huang@...ux.intel.com>, "Mehta, Sohil"
<sohil.mehta@...el.com>, "mingo@...hat.com" <mingo@...hat.com>,
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CC: "mikko.ylinen@...ux.intel.com" <mikko.ylinen@...ux.intel.com>,
"seanjc@...gle.com" <seanjc@...gle.com>, "anakrish@...rosoft.com"
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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 23/02/2024 6:20 am, Haitao Huang wrote:
> On Wed, 21 Feb 2024 05:00:27 -0600, Huang, Kai <kai.huang@...el.com> wrote:
>
>> On Wed, 2024-02-21 at 00:44 -0600, Haitao Huang wrote:
>>> [...]
>>> >
>>> > Here the @nr_to_scan is reduced by the number of pages that are
>>> > isolated, but
>>> > not actually reclaimed (which is reflected by @cnt).
>>> >
>>> > IIUC, looks you want to make this function do "each cycle" as what you
>>> > mentioned
>>> > in the v8 [1]:
>>> >
>>> > I tested with that approach and found we can only target number of
>>> > pages
>>> > attempted to reclaim not pages actually reclaimed due to the
>>> > uncertainty
>>> > of how long it takes to reclaim pages. Besides targeting number of
>>> > scanned pages for each cycle is also what the ksgxd does.
>>> >
>>> > If we target actual number of pages, sometimes it just takes
>>> too long.
>>> > I
>>> > saw more timeouts with the default time limit when running
>>> parallel
>>> > selftests.
>>> >
>>> > I am not sure what does "sometimes it just takes too long" mean, but
>>> > what I am
>>> > thinking is you are trying to do some perfect but yet complicated code
>>> > here.
>>>
>>> I think what I observed was that the try_charge() would block too long
>>> before getting chance of schedule() to yield, causing more timeouts than
>>> necessary.
>>> I'll do some re-test to be sure.
>>
>> Looks this is a valid information that can be used to justify whatever
>> you are
>> implementing in the EPC cgroup reclaiming function(s).
>>
> I'll add some comments. Was assuming this is just following the old
> design as ksgxd.
> There were some comments at the beginning of
> sgx_epc_cgrooup_reclaim_page().
> /*
> * 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.
> */
> unsigned int nr_to_scan = SGX_NR_TO_SCAN;
>
> I think it can be improved to emphasize we only "attempt" to finish
> scanning fixed number of pages for reclamation, not enforce number of
> pages successfully reclaimed.
Not sure need to be this comment, but at somewhere just state you are
trying to follow the ksgxd() (the current sgx_reclaim_pages()), but
trying to do it "_across_ given cgroup and all the descendants".
That's the reason you made @nr_to_scan as a pointer.
And also some text to explain why to follow ksgxd() -- not wanting to
block longer due to loop over descendants etc -- so we can focus on
discussing whether such justification is reasonable.
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