On Sat, Jun 15, 2013 at 09:36:00AM -0400, Peter Hurley wrote: > ** v2 changes ** > - Rebased on v4 of 'lockless n_tty receive path' > > This 2nd of 4 patchsets implements lockless receive from the tty driver. > By lockless, I'm referring to the 'lock' spin lock formerly used to > serialize access to the flip buffer list. > > Since the driver-side flip buffer usage is already single-threaded and > line discipline receiving is already single-threaded, implementing > a lockless flip buffer list was the primary hurdle. [The only 2 flip > buffer consumers, flush_to_ldisc() and tty_buffer_flush() were already > mutually exclusive and this exclusion remains although the mechanism > is changed.] > > Since the flip buffer consumers, flush_to_ldisc() and tty_buffer_flush(), > already leave the last-consumed flip buffer on the list, and since the > existing flip buffer api is already divided into an add/commit interface, > most of the requirement for a lockless algorithm was already > in-place. The main differences are; > 1) the initial state of the flip buffer list points head and tail > to a 0-sized sentinel flip buffer. This eliminates head & tail NULL > testing and assigning the head ptr from the driver-side thread. This > sentinel is 'consumed' on the first iteration of ldisc receiving and > does not require special-case logic. > 2) the free list uses the atomic singly-linked llist interface. While > this guarantees safe concurrent usage by both producer and consumer, > it's not optimal. Both producer and consumer unnecessarily contend > over the free list head ptr; a better approach would be to maintain > an unconsumed buffer in the same way the flip buffer list works. > Light testing has shown this contention accounts for roughly 5% of > total cpu time in end-to-end copying. > 3) The mutual exclusion between consumers is reimplemented as a mutex; > this eliminates the need to drop the lock across the ldisc > receive_buf() method. This mutual exclusion is extended to a public > interface which the vt driver now uses to safely utilize the ldisc > receive_buf() interface when pasting a selection. All applied, thanks. greg k-h -- To unsubscribe from this list: send the line "unsubscribe linux-serial" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
