Messages in this thread |  | | From | "Huang, Ying" <> | | Subject | Re: [RFC PATCH v4 4/6] mm/migrate: add copy offload registration infrastructure | | Date | Thu, 30 Apr 2026 09:23:36 +0800 |
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"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
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