Messages in this thread |  | | Date | Fri, 3 Apr 2026 16:41:06 +0530 | | Subject | Re: [RFC PATCH v4 4/6] mm/migrate: add copy offload registration infrastructure | | From | "Garg, Shivank" <> |
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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.
Best regards, Shivank
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