Re: [RFC PATCH for 4.18 00/14] Restartable Sequences

From: Ben Maurer <bmaurer@fb.com>
To: Joel Fernandes <joelaf@google.com>,
	Daniel Colascione <dancol@google.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Paul McKenney <paulmck@linux.vnet.ibm.com>,
	"Boqun Feng" <boqun.feng@gmail.com>,
	Andy Lutomirski <luto@amacapital.net>,
	"Dave Watson" <davejwatson@fb.com>,
	LKML <linux-kernel@vger.kernel.org>,
	"linux-api@vger.kernel.org" <linux-api@vger.kernel.org>,
	Paul Turner <pjt@google.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	"linux@arm.linux.org.uk" <linux@arm.linux.org.uk>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, "hpa@zytor.com" <hpa@zytor.com>,
	Andrew Hunter <ahh@google.com>,
	"andi@firstfloor.org" <andi@firstfloor.org>,
	"cl@linux.com" <cl@linux.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Josh Triplett <josh@joshtriplett.org>,
	"torvalds@linux-foundation.org" <torvalds@linux-foundation.org>,
	"Catalin Marinas" <catalin.marinas@arm.com>,
	Will Deacon <will.deacon@arm.com>,
	"mtk.manpages@gmail.com" <mtk.manpages@gmail.com>,
	"longman@redhat.com" <longman@redhat.com>
Subject: Re: [RFC PATCH for 4.18 00/14] Restartable Sequences
Date: Fri, 4 May 2018 22:17:38 +0000	[thread overview]
Message-ID: <BL0PR1501MB2067A0682D5D74E040ABC9AECF860@BL0PR1501MB2067.namprd15.prod.outlook.com> (raw)
In-Reply-To: <CAJWu+opkzpmwY5jJ0353JKyaOFCqa95AKGhxUdQZ6pZf_omsaQ@mail.gmail.com>

Hey -

I think the ideas Daniel brings up here are interesting -- specifically the notion that a thread could set a "pre-sleep wish" to signal it's sleeping. As this conversation shows I think there's a fair bit of depth to that. For example, the FUTEX_LOCK is an alternative approach. Another idea might be using the "currently running cpu" area of rseq to tell if the process in question was sleeping (assuming that the kernel would be modified to set this to -1 every time a process was unscheduled)

The idea discussed here seems orthogonal to the core thesis of rseq. I'm wondering if we can focus on getting rseq in, maybe with a eye for making sure this use case could be handled long term. My sense is that this is possible. We could use the flags setting in the per-thread rseq area, or maybe extend the meaning of the structure rseq_cs points to to signal that there was information about how to signal the sleeping of the current process. It seems to me this would be a natural way to add the functionality Daniel talks about if desired in the future.

Daniel - do you think there's anything we should do in the patch set today that would make it easier to implement your idea in the future without expanding the scope of the patch today. i.e. is there anything else we need to do to lay the framework for your idea.

I'd really love to see us get this patch in. There's a whole host of primitives this unlocks (more efficient RCU, better malloc implementations, fast reader-writer locks). I'm sure we'll have more ideas about APIs to provide once we've explored these use cases, but to me this patch is the MVP we need to ship to get that feedback. It's a solid framework that we can build upon, eg with the "opv" syscall or the idea in this thread if user feedback shows those things are necessary.

-b