From: Thierry Delisle <tdelisle@uwaterloo.ca>
To: <posk@posk.io>
Cc: <avagin@google.com>, <bsegall@google.com>, <jannh@google.com>,
<jnewsome@torproject.org>, <joel@joelfernandes.org>,
<linux-api@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
<mingo@redhat.com>, <peterz@infradead.org>, <pjt@google.com>,
<posk@google.com>, <tglx@linutronix.de>,
Peter Buhr <pabuhr@uwaterloo.ca>,
Martin Karsten <mkarsten@uwaterloo.ca>
Subject: Re: [RFC PATCH 3/4 v0.3] sched/umcg: RFC: add userspace sll helpers
Date: Fri, 16 Jul 2021 20:58:09 -0400 [thread overview]
Message-ID: <e84df89d-5c5b-c3fd-42f6-2240b8a110a1@uwaterloo.ca> (raw)
In-Reply-To: <20210716184719.269033-4-posk@google.com>
> The helpers below can work with userspace single-linked lists
concurrently
> without indefinitely spinning in the kernel. Specifically:
>
> push (add to the head of the list):
>
> step = 0
> while (++step < N)
> old = *head
> *next = old
> cmp_xchng(head, &old, next)
>
> pop (remove the first element from the list):
> mark the node as deleted by flipping its lowest bit without
> actually removing the node from the list:
>
> curr = *head
> step = 0
>
> while (curr && ++step < N)
>
> next = *curr
> if (next & 1)
> curr = next & ~1
> continue
>
> if (cmp_xchng(curr, next, next | 1))
> return curr
> else
> curr = next & ~1
>
> It is the userspace's responsibility to actually remove the
> nodes marked as deleted from the list.
I believe the subsystem called Nemesis introduced a MCS-based queue that
could
be useful here. The original paper is https://doi.org/10.1109/CCGRID.2006.31
You can also look at this repo for an implementation of the queue.
https://git.uwaterloo.ca/mkarsten/libfibre/-/blob/master/src/runtime/LockFreeQueues.h
While it uses 2 pointers, I believe it would fit well for idle_workers_ptr
since the push operation is wait-free, which avoids problems for the kernel.
The pop operation requires a lock in userspace *if* multiple servers can
consume from the same queue, but I don't believe it's a requirement in
general.
next prev parent reply other threads:[~2021-07-17 0:58 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-07-16 18:47 [RFC PATCH 0/4 v0.3] sched/UMCG Peter Oskolkov
2021-07-16 18:47 ` [RFC PATCH 1/4 v0.3] sched: add WF_CURRENT_CPU and externise ttwu Peter Oskolkov
2021-07-16 18:47 ` [RFC PATCH 2/4 v0.3] sched/umcg: RFC: add userspace atomic helpers Peter Oskolkov
2021-07-16 18:47 ` [RFC PATCH 3/4 v0.3] sched/umcg: RFC: add userspace sll helpers Peter Oskolkov
2021-07-17 0:58 ` Thierry Delisle [this message]
2021-07-16 18:47 ` [RFC PATCH 4/4 v0.3] sched/umcg: RFC: implement UMCG syscalls Peter Oskolkov
2021-07-19 16:07 ` Thierry Delisle
2021-07-19 17:29 ` Peter Oskolkov
2021-07-19 18:13 ` Thierry Delisle
2021-07-19 19:46 ` Peter Oskolkov
2021-07-21 19:55 ` Thierry Delisle
2021-07-21 23:44 ` Peter Oskolkov
2021-07-23 19:06 ` Thierry Delisle
2021-07-26 16:44 ` Peter Oskolkov
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