Messages in this thread | | | Date | Mon, 6 Jun 2022 22:09:02 +0530 | Subject | Re: [RFC PATCH v4 2/7] mm/demotion: Expose per node memory tier to sysfs | From | Aneesh Kumar K V <> |
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On 6/6/22 9:46 PM, Jonathan Cameron wrote: > On Mon, 6 Jun 2022 21:31:16 +0530 > Aneesh Kumar K V <aneesh.kumar@linux.ibm.com> wrote: > >> On 6/6/22 8:29 PM, Jonathan Cameron wrote: >>> On Fri, 3 Jun 2022 14:10:47 +0530 >>> Aneesh Kumar K V <aneesh.kumar@linux.ibm.com> wrote: >>> >>>> On 5/27/22 7:45 PM, Jonathan Cameron wrote: >>>>> On Fri, 27 May 2022 17:55:23 +0530 >>>>> "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com> wrote: >>>>> >>>>>> From: Jagdish Gediya <jvgediya@linux.ibm.com> >>>>>> >>>>>> Add support to read/write the memory tierindex for a NUMA node. >>>>>> >>>>>> /sys/devices/system/node/nodeN/memtier >>>>>> >>>>>> where N = node id >>>>>> >>>>>> When read, It list the memory tier that the node belongs to. >>>>>> >>>>>> When written, the kernel moves the node into the specified >>>>>> memory tier, the tier assignment of all other nodes are not >>>>>> affected. >>>>>> >>>>>> If the memory tier does not exist, writing to the above file >>>>>> create the tier and assign the NUMA node to that tier. >>>>> creates >>>>> >>>>> There was some discussion in v2 of Wei Xu's RFC that what matter >>>>> for creation is the rank, not the tier number. >>>>> >>>>> My suggestion is move to an explicit creation file such as >>>>> memtier/create_tier_from_rank >>>>> to which writing the rank gives results in a new tier >>>>> with the next device ID and requested rank. >>>> >>>> I think the below workflow is much simpler. >>>> >>>> :/sys/devices/system# cat memtier/memtier1/nodelist >>>> 1-3 >>>> :/sys/devices/system# cat node/node1/memtier >>>> 1 >>>> :/sys/devices/system# ls memtier/memtier* >>>> nodelist power rank subsystem uevent >>>> /sys/devices/system# ls memtier/ >>>> default_rank max_tier memtier1 power uevent >>>> :/sys/devices/system# echo 2 > node/node1/memtier >>>> :/sys/devices/system# >>>> >>>> :/sys/devices/system# ls memtier/ >>>> default_rank max_tier memtier1 memtier2 power uevent >>>> :/sys/devices/system# cat memtier/memtier1/nodelist >>>> 2-3 >>>> :/sys/devices/system# cat memtier/memtier2/nodelist >>>> 1 >>>> :/sys/devices/system# >>>> >>>> ie, to create a tier we just write the tier id/tier index to >>>> node/nodeN/memtier file. That will create a new memory tier if needed >>>> and add the node to that specific memory tier. Since for now we are >>>> having 1:1 mapping between tier index to rank value, we can derive the >>>> rank value from the memory tier index. >>>> >>>> For dynamic memory tier support, we can assign a rank value such that >>>> new memory tiers are always created such that it comes last in the >>>> demotion order. >>> >>> I'm not keen on having to pass through an intermediate state where >>> the rank may well be wrong, but I guess it's not that harmful even >>> if it feels wrong ;) >>> >> >> Any new memory tier added can be of lowest rank (rank - 0) and hence >> will appear as the highest memory tier in demotion order. > > Depends on driver interaction - if new memory is CXL attached or > GPU attached, chances are the driver has an input on which tier > it is put in by default. > >> User can then >> assign the right rank value to the memory tier? Also the actual demotion >> target paths are built during memory block online which in most case >> would happen after we properly verify that the device got assigned to >> the right memory tier with correct rank value? > > Agreed, though that may change the model of how memory is brought online > somewhat. > >> >>> Races are potentially a bit of a pain though depending on what we >>> expect the usage model to be. >>> >>> There are patterns (CXL regions for example) of guaranteeing the >>> 'right' device is created by doing something like >>> >>> cat create_tier > temp.txt >>> #(temp gets 2 for example on first call then >>> # next read of this file gets 3 etc) >>> >>> cat temp.txt > create_tier >>> # will fail if there hasn't been a read of the same value >>> >>> Assuming all software keeps to the model, then there are no >>> race conditions over creation. Otherwise we have two new >>> devices turn up very close to each other and userspace scripting >>> tries to create two new tiers - if it races they may end up in >>> the same tier when that wasn't the intent. Then code to set >>> the rank also races and we get two potentially very different >>> memories in a tier with a randomly selected rank. >>> >>> Fun and games... And a fine illustration why sysfs based 'device' >>> creation is tricky to get right (and lots of cases in the kernel >>> don't). >>> >> >> I would expect userspace to be careful and verify the memory tier and >> rank value before we online the memory blocks backed by the device. Even >> if we race, the result would be two device not intended to be part of >> the same memory tier appearing at the same tier. But then we won't be >> building demotion targets yet. So userspace could verify this, move the >> nodes out of the memory tier. Once it is verified, memory blocks can be >> onlined. > > The race is there and not avoidable as far as I can see. Two processes A and B. > > A checks for a spare tier number > B checks for a spare tier number > A tries to assign node 3 to new tier 2 (new tier created) > B tries to assign node 4 to new tier 2 (accidentally hits existing tier - as this > is the same method we'd use to put it in the existing tier we can't tell this > write was meant to create a new tier). > A writes rank 100 to tier 2 > A checks rank for tier 2 and finds it is 100 as expected. > B write rank 200 to tier 2 (it could check if still default but even that is racy) > B checks rank for tier 2 rank and finds it is 200 as expected. > A onlines memory. > B onlines memory. > > Both think they got what they wanted, but A definitely didn't. > > One work around is the read / write approach and create_tier. > > A reads create_tier - gets 2. > B reads create_tier - gets 3. > A writes 2 to create_tier as that's what it read. > B writes 3 to create_tier as that's what it read. > > continue with created tiers. Obviously can exhaust tiers, but if this is > root only, could just create lots anyway so no worse off. > >> >> Having said that can you outline the usage of >> memtier/create_tier_from_rank ? > > There are corner cases to deal with... > > A writes 100 to create_tier_from_rank. > A goes looking for matching tier - finds it: tier2 > B writes 200 to create_tier_from_rank > B goes looking for matching tier - finds it: tier3 > > rest is fine as operating on different tiers. > > Trickier is > A writes 100 to create_tier_from_rank - succeed. > B writes 100 to create_tier_from_rank - Could fail, or could just eat it? > > Logically this is same as separate create_tier and then a write > of rank, but in one operation, but then you need to search > for the right one. As such, perhaps a create_tier > that does the read/write pair as above is the best solution. >
This all is good when we allow dynamic rank values. But currently we are restricting ourselves to three rank value as below:
rank memtier 300 memtier0 200 memtier1 100 memtier2
Now with the above, how do we define a write to create_tier_from_rank. What should be the behavior if user write value other than above defined rank values? Also enforcing the above three rank values as supported implies teaching userspace about them. I am trying to see how to fit create_tier_from_rank without requiring the above.
Can we look at implementing create_tier_from_rank when we start supporting dynamic tiers/rank values? ie,
we still allow node/nodeN/memtier. But with dynamic tiers a race free way to get a new memory tier would be echo rank > memtier/create_tier_from_rank. We could also say, memtier0/1/2 are kernel defined memory tiers. Writing to memtier/create_tier_from_rank will create new memory tiers above memtier2 with the rank value specified?
-aneesh
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