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Message-ID: <CABk29NsgPcpOpDzixKtdkK85H1xq2XHXHUm6B96nS-4zKzpRwg@mail.gmail.com> Date: Fri, 30 Oct 2020 17:42:12 -0700 From: Josh Don <joshdon@...gle.com> To: "Joel Fernandes (Google)" <joel@...lfernandes.org> Cc: Nishanth Aravamudan <naravamudan@...italocean.com>, Julien Desfossez <jdesfossez@...italocean.com>, Peter Zijlstra <peterz@...radead.org>, Tim Chen <tim.c.chen@...ux.intel.com>, Vineeth Pillai <viremana@...ux.microsoft.com>, Aaron Lu <aaron.lwe@...il.com>, Aubrey Li <aubrey.intel@...il.com>, Thomas Gleixner <tglx@...utronix.de>, linux-kernel <linux-kernel@...r.kernel.org>, mingo@...nel.org, torvalds@...ux-foundation.org, fweisbec@...il.com, keescook@...omium.org, kerrnel@...gle.com, Phil Auld <pauld@...hat.com>, Valentin Schneider <valentin.schneider@....com>, Mel Gorman <mgorman@...hsingularity.net>, Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>, Paolo Bonzini <pbonzini@...hat.com>, vineeth@...byteword.org, Chen Yu <yu.c.chen@...el.com>, Christian Brauner <christian.brauner@...ntu.com>, Agata Gruza <agata.gruza@...el.com>, Antonio Gomez Iglesias <antonio.gomez.iglesias@...el.com>, graf@...zon.com, konrad.wilk@...cle.com, dfaggioli@...e.com, Paul Turner <pjt@...gle.com>, Steven Rostedt <rostedt@...dmis.org>, derkling@...gle.com, benbjiang@...cent.com, Alexandre Chartre <alexandre.chartre@...cle.com>, James.Bottomley@...senpartnership.com, OWeisse@...ch.edu, Dhaval Giani <dhaval.giani@...cle.com>, Junaid Shahid <junaids@...gle.com>, Jesse Barnes <jsbarnes@...gle.com>, chris.hyser@...cle.com, Aubrey Li <aubrey.li@...ux.intel.com>, "Paul E. McKenney" <paulmck@...nel.org>, Tim Chen <tim.c.chen@...el.com>, Benjamin Segall <bsegall@...gle.com>, Hao Luo <haoluo@...gle.com> Subject: Re: [PATCH v8 -tip 19/26] sched: Add a second-level tag for nested CGroup usecase On Mon, Oct 19, 2020 at 6:45 PM Joel Fernandes (Google) <joel@...lfernandes.org> wrote: > > +static unsigned long cpu_core_get_group_cookie(struct task_group *tg) > +{ > + unsigned long color = 0; > + > + if (!tg) > + return 0; > + > + for (; tg; tg = tg->parent) { > + if (tg->core_tag_color) { > + WARN_ON_ONCE(color); > + color = tg->core_tag_color; > + } > + > + if (tg->core_tagged) { > + unsigned long cookie = ((unsigned long)tg << 8) | color; > + cookie &= (1UL << (sizeof(unsigned long) * 4)) - 1; > + return cookie; > + } > + } > + > + return 0; > +} I'm a bit wary of how core_task_cookie and core_group_cookie are truncated to the lower half of their bits and combined into the overall core_cookie. Now that core_group_cookie is further losing 8 bits to color, that leaves (in the case of 32 bit unsigned long) only 8 bits to uniquely identify the group contribution to the cookie. Also, I agree that 256 colors is likely adequate, but it would be nice to avoid this restriction. I'd like to propose the following alternative, which involves creating a new struct to represent the core cookie: struct core_cookie { unsigned long task_cookie; unsigned long group_cookie; unsigned long color; /* can be further extended with arbitrary fields */ struct rb_node node; refcount_t; }; struct rb_root core_cookies; /* (sorted), all active core_cookies */ seqlock_t core_cookies_lock; /* protects against removal/addition to core_cookies */ struct task_struct { ... unsigned long core_cookie; /* (struct core_cookie *) */ } A given task stores the address of a core_cookie struct in its core_cookie field. When we reconfigure a task's color/task_cookie/group_cookie, we can first look for an existing core_cookie that matches those settings, or create a new one.
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