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h2: Make requests walk away from the waiting list #3835

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h2: Make requests walk away from the waiting list #3835

wants to merge 5 commits into from

Commits on Oct 19, 2022

  1. transport: Add a waiting list methods for req_top 

    It comes with new functions hsh_waitlist() and hsh_reembark() that
centralize the logic. The top_waitlist() and top_reembark() methods
are meant for client-facing transport to register when requests and
sub-requests alike enter and leave a waiting list.
    @dridi
    dridi committed Oct 19, 2022
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  2. transport: Remove the vtr_reembark_f typedef 

    Since both vtr_reembark_f and vtr_waitlist_f have the same signature, we
can use vtr_waitlist_f to convey that those callbacks relate to waiting
lists, and the name of the methods to convey their roles.
    @dridi
    dridi committed Oct 19, 2022
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  3. hash: New HSH_WalkAway() function 

    When there's no rush, a request can simply walk away from the queue.

The concept is simple: when a factor (pun intended) other than the
backend fetch requires the request to leave the queue, it can just
walk away. It short-circuits the usual rush exit.

We can repurpose the dead counter busy_killed to count those events,
and get rid of the dead enum entry HSH_CONTINUE.
    @dridi
    dridi committed Oct 19, 2022
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  4. h2: Keep track of streams entering waiting lists 

    But operate on a different objhead reference guarded by the h2_sess lock
instead. This will allow coordination with the h2 session thread.
    @dridi
    dridi committed Oct 19, 2022
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  5. h2: Make requests walk away from the waiting list 

    When a session goes to the cleanup step, it will indefinitely try to
kill all the streams until those stuck in a waiting list reembark a
thread. This can lead to clients timing out, closing the h2 session,
and opening a new h2 session for the exact same requests. This can
artifically inflate the number of dead tasks in a waiting list with
generous backend timeouts.

To avoid that, an h2 stream will keep a list of requests currently
in a waiting list and make them walk away quietly during cleanup.
An h2 stream would normally only have one request running at any
time but this may no longer be true with modules like vmod_pesi.

Better diff with the --ignore-all-space option.
    @dridi
    dridi committed Oct 19, 2022
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