Messages in this thread |  | | Date | Tue, 28 Apr 2026 17:40:53 +0530 | | Subject | Re: [RFC PATCH v4 4/6] mm/migrate: add copy offload registration infrastructure | | From | "Garg, Shivank" <> |
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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.
Thanks, Shivank
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