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a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1600864398; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=4zUEMW9wU+kwouVfGxKhtf6Di/IFhmYOqDGQKAREV8o=; b=ohpCeECCtFRlt7dpUmuCjEyhCy6FZxCGdV74xdJ5JgvLxUGaba5R+t2Tfw4DYzdeiW9o4A 74IrdXtsMBTtycCw== To: peterz@infradead.org Subject: Re: [patch RFC 00/15] mm/highmem: Provide a preemptible variant of kmap_atomic & friends In-Reply-To: <20200923101953.GT2674@hirez.programming.kicks-ass.net> 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> <20200923101953.GT2674@hirez.programming.kicks-ass.net> Date: Wed, 23 Sep 2020 14:33:17 +0200 Message-ID: <87sgb8ad0y.fsf@nanos.tec.linutronix.de> MIME-Version: 1.0 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20200923_083320_077427_A143970B X-CRM114-Status: GOOD ( 17.02 ) X-BeenThere: linux-snps-arc@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on Synopsys ARC Processors List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Juri Lelli , David Airlie , Benjamin Herrenschmidt , Sebastian Andrzej Siewior , Joonas Lahtinen , dri-devel , linux-mips@vger.kernel.org, Ben Segall , Max Filippov , Guo Ren , linux-sparc , Vincent Chen , Will Deacon , Ard Biesheuvel , linux-arch , Vincent Guittot , Herbert Xu , Michael Ellerman , the arch/x86 maintainers , Russell King , linux-csky@vger.kernel.org, Mel Gorman , "open list:SYNOPSYS ARC ARCHITECTURE" , linux-xtensa@linux-xtensa.org, Paul McKenney , intel-gfx , linuxppc-dev , Steven Rostedt , Jani Nikula , Rodrigo Vivi , Dietmar Eggemann , Linux ARM , Chris Zankel , Michal Simek , Thomas Bogendoerfer , Nick Hu , Linux-MM , Linus Torvalds , LKML , Arnd Bergmann , Daniel Vetter , Vineet Gupta , Paul Mackerras , Andrew Morton , Daniel Bristot de Oliveira , "David S. Miller" , Greentime Hu Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-snps-arc" Errors-To: linux-snps-arc-bounces+linux-snps-arc=archiver.kernel.org@lists.infradead.org On Wed, Sep 23 2020 at 12:19, peterz wrote: > On Mon, Sep 21, 2020 at 09:27:57PM +0200, Thomas Gleixner wrote: >> Alternatively this could of course be solved with per CPU page tables >> which will come around some day anyway I fear. > > Previously (with PTI) we looked at making the entire kernel map per-CPU, > and that takes a 2K copy on switch_mm() (or more general, the user part > of whatever the top level directory is for architectures that have a > shared kernel/user page-table setup in the first place). > > The idea was having a fixed per-cpu kernel page-table, share a bunch of > (kernel) page-tables between all CPUs and then copy in the user part on > switch. > > I've forgotten what the plan was for ASID/PCID in that scheme. > > For x86_64 we've been fearing the performance of that 2k copy, but I > don't think we've ever actually bit the bullet and implemented it to see > how bad it really is. I actually did at some point and depending on the workload the overhead was clearly measurable. And yes, it fell apart with PCID and I could not come up with a scheme for it which did not suck horribly. So I burried the patches in the poison cabinet. Aside of that, we'd need to implement that for a eight other architectures as well... Thanks, tglx _______________________________________________ linux-snps-arc mailing list linux-snps-arc@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-snps-arc