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[5.38.241.3]) by smtp.gmail.com with ESMTPSA id hq29sm10982893ejc.141.2022.01.03.08.29.03 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 03 Jan 2022 08:29:04 -0800 (PST) Sender: Ingo Molnar Date: Mon, 3 Jan 2022 17:29:02 +0100 From: Ingo Molnar To: Greg Kroah-Hartman Cc: Linus Torvalds , linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, Andrew Morton , Peter Zijlstra , Thomas Gleixner , "David S. Miller" , Ard Biesheuvel , Josh Poimboeuf , Jonathan Corbet , Al Viro Subject: Re: [PATCH 0000/2297] [ANNOUNCE, RFC] "Fast Kernel Headers" Tree -v1: Eliminate the Linux kernel's "Dependency Hell" Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Greg Kroah-Hartman wrote: > > The overall policy to extend task_struct, going forward, would be to: > > > > - Either make simple-type or struct-pointer additions to task_struct, that > > don't couple to other subsystems. > > > > - Or, if you absolutely must - and we don't want to forbid this - use the > > per_task() machinery to create a simple accessor to a complex embedded > > type. > > I'll leave all of this up to the scheduler developers, but it still looks > odd to me. The mess we create trying to work around issues in C :) Yeah, so I *did* find this somewhat suboptimal too, and developed an earlier version that used linker section tricks to gain the field offsets more automatically. It was an unmitigated disaster: was fragile on x86 already (which has a zoo of linking quirks with no precedent of doing this before bounds.c processing), but on ARM64 and probably on most of the other RISC-ish architectures there was also a real runtime code generation cost of using linker tricks: 2-3 extra instructions per per_task() use - clearly unacceptable. Found this out the hard way after making it boot & work on ARM64 and looking at the assembly output, trying to figure out why the generated code size increased. :-/ Anyway, the current method has the big advantage of being obviously invariant wrt. code generation compared to the previous code, on every architecture. > > Do these plans sound good to you? > > Yes, taking the majority through the maintainer trees and then doing the > remaining bits in a single tree seems sane, that one tree will be easier > to review as well. Ok. Will definitely offer it up piecemail-wise, in reviewable chunks, via existing processes & flows. Thanks, Ingo