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Miller" , linux-sparc Subject: Re: [patch RFC 00/15] mm/highmem: Provide a preemptible variant of kmap_atomic & friends In-Reply-To: <20200923171234.0001402d@oasis.local.home> References: <20200919091751.011116649@linutronix.de> <87mu1lc5mp.fsf@nanos.tec.linutronix.de> <87k0wode9a.fsf@nanos.tec.linutronix.de> <87eemwcpnq.fsf@nanos.tec.linutronix.de> <87a6xjd1dw.fsf@nanos.tec.linutronix.de> <87sgbbaq0y.fsf@nanos.tec.linutronix.de> <20200923084032.GU1362448@hirez.programming.kicks-ass.net> <20200923115251.7cc63a7e@oasis.local.home> <874kno9pr9.fsf@nanos.tec.linutronix.de> <20200923171234.0001402d@oasis.local.home> Date: Thu, 24 Sep 2020 08:57:52 +0200 Message-ID: <871riracgf.fsf@nanos.tec.linutronix.de> MIME-Version: 1.0 Content-Type: text/plain X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, Sep 23 2020 at 17:12, Steven Rostedt wrote: > On Wed, 23 Sep 2020 22:55:54 +0200 > Then scratch the idea of having anonymous local_lock() and just bring > local_lock in directly? Then have a kmap local lock, which would only > block those that need to do a kmap. That's still going to end up in lock ordering nightmares and you lose the ability to use kmap_local from arbitrary contexts which was again one of the goals of this exercise. Aside of that you're imposing reentrancy protections on something which does not need it in the first place. > Now as for migration disabled nesting, at least now we would have > groupings of this, and perhaps the theorists can handle that. I mean, > how is this much different that having a bunch of tasks blocked on a > mutex with the owner is pinned on a CPU? > > migrate_disable() is a BKL of pinning affinity. No. That's just wrong. preempt disable is a concurrency control, i.e. protecting against reentrancy on a given CPU. But it's a cpu global protection which means that it's not protecting a specific code path. Contrary to preempt disable, migrate disable is not protecting against reentrancy on a given CPU. It's a temporary restriction to the scheduler on placement. The fact that disabling preemption implicitely disables migration does not make them semantically equivalent. > If we only have local_lock() available (even on !RT), then it makes > the blocking in groups. At least this way you could grep for all the > different local_locks in the system and plug that into the algorithm > for WCS, just like one would with a bunch of mutexes. You cannot do that on RT at all where migrate disable is substituting preempt disable in spin and rw locks. The result would be the same as with a !RT kernel just with horribly bad performance. That means the stacking problem has to be solved anyway. So why on earth do you want to create yet another special duct tape case for kamp_local() which proliferates inconsistency instead of aiming for consistency accross all preemption models? Thanks, tglx