Re: [RFC PATCH v2 4/5] sched: UMCG: add a blocked worker list
From: Peter Zijlstra <peterz@infradead.org>
Date: 2022-01-27 15:38:20
Also in:
linux-mm, lkml
On Thu, Jan 13, 2022 at 03:39:39PM -0800, Peter Oskolkov wrote:
This change introduces the following benefits: - block detection how behaves similarly to wake detection; without this patch worker wakeups added wakees to the list and woke the server, while worker blocks only woke the server without adding blocked workers to a list, forcing servers to explicitly check worker's state;
- if the blocked worker woke sufficiently quickly, the server woken on the block event would observe its worker now as RUNNABLE, so the block event had to be inferred rather than explicitly signalled by the worker being added to the blocked worker list;
This I think is missing the point, there is no race if the server checks curr->state == RUNNING.
- it is now possible for a single server to control several RUNNING workers, which makes writing userspace schedulers simpler for smaller processes that do not need to scale beyond one "server";
How about something like so on top?
--- a/include/linux/sched.h
+++ b/include/linux/sched.h@@ -1298,6 +1298,7 @@ struct task_struct { #ifdef CONFIG_UMCG /* setup by sys_umcg_ctrl() */ + u32 umcg_flags; clockid_t umcg_clock; struct umcg_task __user *umcg_task; --- a/include/uapi/linux/umcg.h +++ b/include/uapi/linux/umcg.h
@@ -119,6 +119,8 @@ struct umcg_task { * * Readable/writable by both the kernel and the userspace: the * kernel adds items to the list, userspace removes them. + * + * Only used with UMCG_CTL_MULTI. */ __u64 blocked_workers_ptr; /* r/w */
@@ -147,11 +149,13 @@ enum umcg_wait_flag { * @UMCG_CTL_REGISTER: register the current task as a UMCG task * @UMCG_CTL_UNREGISTER: unregister the current task as a UMCG task * @UMCG_CTL_WORKER: register the current task as a UMCG worker + * @UMCG_CTL_MULTI: allow 1:n worker relations, enables blocked_workers_ptr */ enum umcg_ctl_flag { UMCG_CTL_REGISTER = 0x00001, UMCG_CTL_UNREGISTER = 0x00002, UMCG_CTL_WORKER = 0x10000, + UMCG_CTL_MULTI = 0x20000, }; #endif /* _UAPI_LINUX_UMCG_H */ --- a/kernel/sched/umcg.c +++ b/kernel/sched/umcg.c
@@ -335,7 +335,7 @@ static inline int umcg_enqueue_runnable( } /* - * Enqueue @tsk on it's server's blocked list + * Enqueue @tsk on it's server's blocked list OR ensure @tsk == server::next_tid * * Must be called in umcg_pin_pages() context, relies on tsk->umcg_server. *
@@ -346,10 +346,34 @@ static inline int umcg_enqueue_runnable( * Returns: * 0: success * -EFAULT + * -ESRCH server::next_tid is not a valid UMCG task + * -EINVAL server::next_tid doesn't match @tsk */ static inline int umcg_enqueue_blocked(struct task_struct *tsk) { - return umcg_enqueue(tsk, true /* blocked */); + struct task_struct *next; + u32 next_tid; + int ret; + + if (tsk->umcg_server->umcg_flags & UMCG_CTL_MULTI) + return umcg_enqueue(tsk, true /* blocked */); + + /* + * When !MULTI, ensure this worker is the current worker, + * ensuring the 1:1 relation. + */ + if (get_user(next_tid, &tsk->umcg_server_task->next_tid)) + return -EFAULT; + + next = umcg_get_task(next_tid); + if (!next) + return -ESRCH; + + ret = (next == tsk) ? 0 : -EINVAL; + + put_task_struct(next); + + return ret; } /* pre-schedule() */
@@ -911,6 +934,8 @@ static int umcg_register(struct umcg_tas return -EINVAL; } + current->umcg_flags = flags; + if (current->umcg_task || !self) return -EINVAL;
@@ -1061,7 +1086,7 @@ SYSCALL_DEFINE3(umcg_ctl, u32, flags, st flags &= ~UMCG_CTL_CMD; - if (flags & ~(UMCG_CTL_WORKER)) + if (flags & ~(UMCG_CTL_WORKER|UMCG_CTL_MULTI)) return -EINVAL; switch (cmd) {