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

["Huang, Ying" <ying.huang@linux.alibaba.com>]

"Garg, Shivank" <shivankg@amd.com> writes:

> On 4/3/2026 4:41 PM, Garg, Shivank wrote:
>> 
>> 
>> On 3/24/2026 4:24 PM, Huang, Ying wrote:
>>> Shivank Garg <shivankg@amd.com> 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?
>> 
>>>
>>>> +#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.
>> 
>>>>  
>>>> +#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.
>
> Coming back to this while reworking the patch.
> I think the static branch guard here is actually redundant. We already check it at
> the per-folio classification that builds src_batch,
> so when offload is disabled src_batch stays empty and the list_empty() check
> short-circuits. I'll still wrap the SRCU + static_call into a helper at this call
> site, as you suggested. Sorry for the flip-flop.

Yes.  It's functionally redundant.  I just want to know whether it can
benefit performance (in a minor way).

---
Best Regards,
Huang, Ying