Re: [PATCH v10 09/25] x86: refactor psr: L3 CAT: set value: implement framework.

[Yi Sun <yi.y.sun@linux.intel.com>]

On 17-04-13 04:58:06, Jan Beulich wrote:
> >>> On 13.04.17 at 12:49, <yi.y.sun@linux.intel.com> wrote:
> > On 17-04-13 03:41:44, Jan Beulich wrote:
> >> >>> On 13.04.17 at 10:11, <yi.y.sun@linux.intel.com> wrote:
> >> > On 17-04-12 06:42:01, Jan Beulich wrote:
> >> >> >>> On 12.04.17 at 14:23, <yi.y.sun@linux.intel.com> wrote:
> >> >> > On 17-04-12 03:09:56, Jan Beulich wrote:
> >> >> >> >>> On 12.04.17 at 07:53, <yi.y.sun@linux.intel.com> wrote:
> >> >> >> > On 17-04-11 09:01:53, Jan Beulich wrote:
> >> >> >> >> >>> On 01.04.17 at 15:53, <yi.y.sun@linux.intel.com> wrote:
> >> >> >> >> Furthermore I'm not at all convinced this is appropriate to do in the
> >> >> >> >> context of a CPU_UP_CANCELED / CPU_DEAD notification: If you
> >> >> >> >> have a few thousand VMs, the loop above may take a while.
> >> >> >> >> 
> >> >> >> > Hmm, that may be a potential issue. I have two proposals below. Could you
> >> >> >> > please help to check which one you prefer? Or provide another solution?
> >> >> >> > 
> >> >> >> > 1. Start a tasklet in free_socket_resources() to restore 
> >> >> > 'psr_cos_ids[socket]'
> >> >> >> >    of all domains. The action is protected by 'ref_lock' to avoid 
> >> >> > confliction
> >> >> >> >    in 'psr_set_val'. We can reduce 'info->cos_ref[cos]' in tasklet or 
> > memset
> >> >> >> >    the array to 0 in free_socket_resources().
> >> >> >> > 
> >> >> >> > 2. Move 'psr_cos_ids[]' from 'domain' to 'psr_socket_info' and change 
> > index
> >> >> >> >    from 'socket' to 'domain_id'. So we keep all domains' COS IDs per 
> > socket
> >> >> >> >    and can memset the array to 0 when socket is offline. But here is an 
> >> >> > issue
> >> >> >> >    that we do not know how many members this array should have. I cannot 
> >> >> > find
> >> >> >> >    a macro something like 'DOMAIN_MAX_NUMBER'. So, I prefer to use 
> >> >> > reallocation
> >> >> >> >    in 'psr_alloc_cos' if the newly created domain's id is bigger than 
> >> >> > current
> >> >> >> >    array number.
> >> >> >> 
> >> >> >> The number of domains is limited by the special DOMID_* values.
> >> >> >> However, allocating an array with 32k entries doesn't sound very
> >> >> >> reasonable.
> >> >> > 
> >> >> > I think 32K entries should be the extreme case. I can allocate e.g. 100 
> > entries
> >> >> > when the first domain is created. If a new domain's id exceeds 100, 
> > reallocate
> >> >> > another 100 entries. The total number of entries allocated should be less 
> > than
> >> >> > 32K. This is a functional requirement which cannot be avoided. How do you 
> >> >> > think?
> >> >> 
> >> >> So how many entries would your array have once I start the 32,000th
> >> >> domain (having at any one time at most a single one running, besides
> >> >> Dom0)?
> >> >> 
> >> > In such case, we have to keep a 32K array because the domain_id is the 
> > index to
> >> > access the array. But this array is per socket so the whole memory used 
> > should
> >> > not be too much.
> >> 
> >> We carefully avoid any runtime allocations of order > 0, so if you
> >> were to set up such an array, you'd need to use vmalloc()/vzalloc().
> >> But I continue to be unconvinced that we want such a large array
> >> in the first place.
> >> 
> >> > After considering this issue more, I think the original codes might not be
> >> > so unacceptable. Per my knowledge, Intel Xeon Phi chip can support at most
> >> > 288 CPUs. So, I think the domains running at same time in reality may not 
> > be
> >> > so many (no efficient resources). If this hypothesis is right, a loop to 
> > write
> >> > 'psr_cos_ids[socket]' of every domain to 0 may not take much time. If I am
> >> > wrong, please correct me. Thanks!
> >> 
> >> What relationship does the number of CPUs have to the number of
> >> domains on a host? There could be thousands with just a few dozen
> >> CPUs, provided none or very few of them have high demands on
> >> CPU resources. Additionally please never forget that system sizes
> >> basically only ever grow. Plus we wouldn't want a latent issue here
> >> in case we ever end up needing to widen domain IDs beyond 16 bits.
> >> 
> > How about a per socket array like this:
> > uint32_t domain_switch[1024];
> > 
> > Every bit represents a domain id. Then, we can handle this case as below:
> > 1. In 'psr_cpu_init()', clear the array to be 0. I think this place is enough to
> >    cover socket offline case. We do not need to clear it in 
> > 'free_socket_resources'.
> > 
> > 2. In 'psr_ctxt_switch_to()', test_and_set_bit(domain_id, domain_switch) to set
> >    the bit to 1 according to domain_id. If the old value is 0 and the 
> >    'psr_cos_ids[socket]' is not 0, restore 'psr_cos_ids[socket]' to be 0.
> > 
> > 3. In 'psr_set_val()', test_and_set_bit(domain_id, domain_switch) to set the bit
> >    to 1 too. Then, update 'psr_cos_ids[socket]' according to find/pick flow.
> > 
> > Then, we only use 4KB for one socket.
> 
> This looks to come closer to something I'd consider acceptable, but
> I may not understand your intentions in full yet: For one, there's
> nowhere you clear the bit (other than presumably during socket
> cleanup). 

Actually, clear the array in 'free_socket_resources' has same effect. I can
move clear action into it.

> And then I don't understand the test_and_ parts of the
> constructs above, i.e. you don't clarify what the return values
> would be used/needed for.
> 
Sorry, 0 means this domain has not been scheduled to the socket yet. If
test_and_ returns 0, that is the first time the domain runs on the socket
(the first time the socket is online). So, we need restore 'psr_cos_ids[socket]'
to 0 in 'psr_ctxt_switch_to()'.

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