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    SubjectRe: [RFC PATCH 79/86] treewide: net: remove cond_resched()
    On Wed, Nov 8, 2023 at 12:09 AM Ankur Arora <ankur.a.arora@oracle.com> wrote:
    >
    > There are broadly three sets of uses of cond_resched():
    >
    > 1. Calls to cond_resched() out of the goodness of our heart,
    > otherwise known as avoiding lockup splats.
    >
    > 2. Open coded variants of cond_resched_lock() which call
    > cond_resched().
    >
    > 3. Retry or error handling loops, where cond_resched() is used as a
    > quick alternative to spinning in a tight-loop.
    >
    > When running under a full preemption model, the cond_resched() reduces
    > to a NOP (not even a barrier) so removing it obviously cannot matter.
    >
    > But considering only voluntary preemption models (for say code that
    > has been mostly tested under those), for set-1 and set-2 the
    > scheduler can now preempt kernel tasks running beyond their time
    > quanta anywhere they are preemptible() [1]. Which removes any need
    > for these explicitly placed scheduling points.

    What about RCU callbacks ? cond_resched() was helping a bit.

    >
    > The cond_resched() calls in set-3 are a little more difficult.
    > To start with, given it's NOP character under full preemption, it
    > never actually saved us from a tight loop.
    > With voluntary preemption, it's not a NOP, but it might as well be --
    > for most workloads the scheduler does not have an interminable supply
    > of runnable tasks on the runqueue.
    >
    > So, cond_resched() is useful to not get softlockup splats, but not
    > terribly good for error handling. Ideally, these should be replaced
    > with some kind of timed or event wait.
    > For now we use cond_resched_stall(), which tries to schedule if
    > possible, and executes a cpu_relax() if not.
    >
    > Most of the uses here are in set-1 (some right after we give up a
    > lock or enable bottom-halves, causing an explicit preemption check.)
    >
    > We can remove all of them.

    A patch series of 86 is not reasonable.

    596 files changed, 881 insertions(+), 2813 deletions(-)

    If really cond_resched() becomes a nop (Nice !) ,
    make this at the definition of cond_resched(),
    and add there nice debugging.

    Whoever needs to call a "real" cond_resched(), could call a
    cond_resched_for_real()
    (Please change the name, this is only to make a point)

    Then let the removal happen whenever each maintainer decides, 6 months
    later, without polluting lkml.

    Imagine we have to revert this series in 1 month, how painful this
    would be had we removed
    ~1400 cond_resched() calls all over the place, with many conflicts.

    Thanks

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