Date: Wed, 03 Jan 2024 14:03:09 +0800
From: "Huang, Ying" <ying.huang@...el.com>
To: Gregory Price <gregory.price@...verge.com>
Cc: Gregory Price <gourry.memverge@...il.com>,  <linux-mm@...ck.org>,
  <linux-doc@...r.kernel.org>,  <linux-fsdevel@...r.kernel.org>,
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Subject: Re: [PATCH v5 01/11] mm/mempolicy: implement the sysfs-based
 weighted_interleave interface

Gregory Price <gregory.price@...verge.com> writes:

> On Wed, Jan 03, 2024 at 10:45:53AM +0800, Huang, Ying wrote:
>> 
>> > The minimum functionality is everything receiving a default weight of 1,
>> > such that weighted interleave's behavior defaults to round-robin
>> > interleave. This gets the system off the ground.
>> 
>> I don't think that we need to implement all functionalities now.  But,
>> we may need to consider more especially if it may impact the user space
>> interface.  The default base weight is something like that.  If we
>> change the default base weight from "1" to "16" later, users may be
>> surprised.  So, I think it's better to discuss it now.
>>
>
> This is a hill I don't particularly care to die on.  I think the weights
> are likely to end up being set at boot and rebalanced as (rare) hotplug
> events occur.
>
> So if people think the default weight should be 3,16,24 or 123, i don't
> think it's going to matter.
>
>> 
>> We can use a wrapper function to hide the logic.
>>
>
> Done.  I'll push a new set tomorrow.
>
>> > I think it also allows MPOL_F_GWEIGHT to be eliminated.
>> 
>> Do we need a way to distinguish whether to copy the global weights to
>> local weights when the memory policy is created?  That is, when the
>> global weights are changed later, will the changes be used?  One
>> possible solution is
>> 
>> - If no weights are specified in set_mempolicy2(), the global weights
>>   will be used always.
>> 
>> - If at least one weight is specified in set_mempolicy2(), it will be
>>   used, and the other weights in global weights will be copied to the
>>   local weights.  That is, changes to the global weights will not be
>>   used.
>> 
>
> What's confusing about that is that if a user sets a weight to 0,
> they'll get a non-0 weight - always.
>
> In my opinion, if we want to make '0' mean 'use system default', then
> it should mean 'ALWAYS use system default for this node'.
>
> "Use the system default at the time the syscall was called, and do not
> update to use a new system default if that default is changed" is
> confusing.
>
> If you say use a global value, use the global value. Simple.

I mainly have concerns about consistency.  The global weights can be
changed while the local weights are fixed.  For example,

- Weights of node 0,1 is [3, 1] initially

- Process A call set_mempolicy2() to set weights to [4, 0], that is, use
  default weight for node 1.

- After hotplug, the weights of node is changed to [12, 4, 1], now the
  effective weights used in process A becomes [4, 4].  Which is hardly
  desired.

Another choice is to disallow "0" as weight in set_mempolicy2().

--
Best Regards,
Huang, Ying

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