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Message-ID: <20190429033620.GA128241@aaronlu> Date: Mon, 29 Apr 2019 11:36:22 +0800 From: Aaron Lu <aaron.lu@...ux.alibaba.com> To: Vineeth Remanan Pillai <vpillai@...italocean.com> Cc: Nishanth Aravamudan <naravamudan@...italocean.com>, Julien Desfossez <jdesfossez@...italocean.com>, Peter Zijlstra <peterz@...radead.org>, Tim Chen <tim.c.chen@...ux.intel.com>, mingo@...nel.org, tglx@...utronix.de, pjt@...gle.com, torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org, subhra.mazumdar@...cle.com, fweisbec@...il.com, keescook@...omium.org, kerrnel@...gle.com, Phil Auld <pauld@...hat.com>, Aaron Lu <aaron.lwe@...il.com>, Aubrey Li <aubrey.intel@...il.com>, Valentin Schneider <valentin.schneider@....com>, Mel Gorman <mgorman@...hsingularity.net>, Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>, Paolo Bonzini <pbonzini@...hat.com> Subject: Re: [RFC PATCH v2 11/17] sched: Basic tracking of matching tasks On Tue, Apr 23, 2019 at 04:18:16PM +0000, Vineeth Remanan Pillai wrote: > +/* > + * l(a,b) > + * le(a,b) := !l(b,a) > + * g(a,b) := l(b,a) > + * ge(a,b) := !l(a,b) > + */ > + > +/* real prio, less is less */ > +static inline bool __prio_less(struct task_struct *a, struct task_struct *b, bool core_cmp) > +{ > + u64 vruntime; > + > + int pa = __task_prio(a), pb = __task_prio(b); > + > + if (-pa < -pb) > + return true; > + > + if (-pb < -pa) > + return false; > + > + if (pa == -1) /* dl_prio() doesn't work because of stop_class above */ > + return !dl_time_before(a->dl.deadline, b->dl.deadline); > + > + vruntime = b->se.vruntime; > + if (core_cmp) { > + vruntime -= task_cfs_rq(b)->min_vruntime; > + vruntime += task_cfs_rq(a)->min_vruntime; > + } > + if (pa == MAX_RT_PRIO + MAX_NICE) /* fair */ > + return !((s64)(a->se.vruntime - vruntime) <= 0); > + > + return false; > +} This unfortunately still doesn't work. Consider the following task layout on two sibling CPUs(cpu0 and cpu1): rq0.cfs_rq rq1.cfs_rq | | se_bash se_hog se_hog is the sched_entity for a cpu intensive task and se_bash is the sched_entity for bash. There are two problems: 1 SCHED_DEBIT when user execute some commands through bash, say ls, bash will fork. The newly forked ls' vruntime is set in the future due to SCHED_DEBIT. This made 'ls' lose in __prio_less() when compared with hog, whose vruntime may very likely be the same as its cfs_rq's min_vruntime. This is OK since we do not want forked process to starve already running ones. The problem is, since hog keeps running, its vruntime will always sync with its cfs_rq's min_vruntime. OTOH, 'ls' can not run, its cfs_rq's min_vruntime doesn't proceed, making 'ls' always lose to hog. 2 who schedules, who wins so I disabled SCHED_DEBIT, for testing's purpose. When cpu0 schedules, ls could win where both sched_entity's vruntime is the same as their cfs_rqs' min_vruntime. So does hog: when cpu1 schedules, hog can preempt ls in the same way. The end result is, interactive task can lose to cpu intensive task and ls can feel "dead". I haven't figured out a way to solve this yet. A core wide cfs_rq's min_vruntime can probably solve this. Your suggestions are appreciated.
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