3a81fd0204
Instead of being pessimistic and assume that path lookup will block, use LOOKUP_CACHED to attempt just a cached lookup. This ensures that the fast path is always done inline, and we only punt to async context if IO is needed to satisfy the lookup. For forced nonblock open attempts, mark the file O_NONBLOCK over the actual ->open() call as well. We can safely clear this again before doing fd_install(), so it'll never be user visible that we fiddled with it. This greatly improves the performance of file open where the dentry is already cached: ached 5.10-git 5.10-git+LOOKUP_CACHED Speedup --------------------------------------------------------------- 33% 1,014,975 900,474 1.1x 89% 545,466 292,937 1.9x 100% 435,636 151,475 2.9x The more cache hot we are, the faster the inline LOOKUP_CACHED optimization helps. This is unsurprising and expected, as a thread offload becomes a more dominant part of the total overhead. If we look at io_uring tracing, doing an IORING_OP_OPENAT on a file that isn't in the dentry cache will yield: 275.550481: io_uring_create: ring 00000000ddda6278, fd 3 sq size 8, cq size 16, flags 0 275.550491: io_uring_submit_sqe: ring 00000000ddda6278, op 18, data 0x0, non block 1, sq_thread 0 275.550498: io_uring_queue_async_work: ring 00000000ddda6278, request 00000000c0267d17, flags 69760, normal queue, work 000000003d683991 275.550502: io_uring_cqring_wait: ring 00000000ddda6278, min_events 1 275.550556: io_uring_complete: ring 00000000ddda6278, user_data 0x0, result 4 which shows a failed nonblock lookup, then punt to worker, and then we complete with fd == 4. This takes 65 usec in total. Re-running the same test case again: 281.253956: io_uring_create: ring 0000000008207252, fd 3 sq size 8, cq size 16, flags 0 281.253967: io_uring_submit_sqe: ring 0000000008207252, op 18, data 0x0, non block 1, sq_thread 0 281.253973: io_uring_complete: ring 0000000008207252, user_data 0x0, result 4 shows the same request completing inline, also returning fd == 4. This takes 6 usec. Signed-off-by: Jens Axboe <axboe@kernel.dk>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.