Re: [RFC PATCH v4 4/6] mm/migrate: add copy offload registration infrastructure

[Huang, Ying]

"Garg, Shivank" <shivankg@xxxxxxx> writes:

> On 3/24/2026 4:24 PM, Huang, Ying wrote:
>> Shivank Garg <shivankg@xxxxxxx> writes:
>> 
>>> Introduce CONFIG_MIGRATION_COPY_OFFLOAD, which lets offload driver
>> 
>> Do we really need a new kconfig option?  IMHO, we have too many now.
>> Because we have a jump label already, the performance difference should
>> be trivial.  Can you measure the size difference?
>
> BASELINE (offload=n)
>    text    data     bss     dec  filename
>   23577    1632      32   25241  mm/migrate.o
> 39202900        14159750        6502152 59864802     vmlinux
>
> WITH OFFLOAD (offload=y)
>    text    data     bss     dec  filename
>   24444    2568      32   27044  mm/migrate.o
>     676      64       8     748  mm/migrate_copy_offload.o
> 39208218        14163942        6498120 59870280     vmlinux
>
> WITHOUT CONFIG (always-on)
>    text    data     bss     dec  filename
>   24444    2568      32   27044  mm/migrate.o
>     676      64       8     748  mm/migrate_copy_offload.o
> 39208405        14163942        6498120 59870467     vmlinux
>
> It saves around 5.5KB of size, when offload support is disabled.
> Is it meaningful savings? What do you think?

The size difference of "vmlinux" is 5.5KB.  While that of *.o is 2.55k.
Not too big for me.

>From another point of view, if we will add kconfig for "migrator"
implementations, we can make this general kconfig option invisible and
be selected automatically?

>> 
>>> +#ifdef CONFIG_MIGRATION_COPY_OFFLOAD
>>> +extern struct static_key_false migrate_offload_enabled;
>>> +extern struct srcu_struct migrate_offload_srcu;
>>> +bool migrate_should_batch_default(int reason);
>>> +int migrate_offload_start(struct migrator *m);
>>> +int migrate_offload_stop(struct migrator *m);
>> 
>> Why not naming the function migrate_offload_register/unregister()?
>> IMHO, that sounds more natural.
>
> Ack. I'll rename to migrate_offload_register/unregister().
>
>> 
>>>  
>>> +#ifdef CONFIG_MIGRATION_COPY_OFFLOAD
>>> +	/* Check if the offload driver wants to batch for this reason */
>>> +	if (static_branch_unlikely(&migrate_offload_enabled))
>>> +		do_batch = static_call(migrate_should_batch)(reason);
>> 
>> Should batching based on "reason" be determined by the general migrate
>> code instead of the migrator implementation?  For example, if we only
>> batch copying for ASYNC migration, we should determine that in
>> migrate_pages_batch() instead of the migreation implementation.  Or am I
>> missed something?  If so, can you provide an example?
>> 
>
> My idea was that different drivers may have different cost/benefit
> profiles(e.g. setup cost, migrate batch-size, etc..) 
>
> For instance, a DMA driver may want to target only bulk migration usecase.
> And a CPU-thread based driver can be used more broadly, without worrying
> about setup-costs.
>
> But I agree it's premature with only one-driver.
> I'll move the reason check with target usecases into migrate_pages_batch()
> and drop the should_batch() callback. If a future driver needs different
> filtering, we can add it back then.

In general, I think that "reason" based policy should be in the general
migrate_pages function.  While "batch size" based policy can be in the
migrator implementations.

>>>  
>>> +#ifdef CONFIG_MIGRATION_COPY_OFFLOAD
>>>  	/* Batch-copy eligible folios before the move phase */
>>>  	if (!list_empty(&src_batch)) {
>> 
>> Guard with "static_branch_unlikely(&migrate_offload_enabled)" first?
>> Better to define a inline function to shorten the expression.
>> 
>
> Sure, will add the static_branch_unlikely guard and wrap in a helper
> function. Thanks.

---
Best Regards,
Huang, Ying