linux-kernel - Re: [RFC PATCH 1/5] x86: introduce preemption disable prefix

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Message-ID: <20181019050051.bs2kk4sagu7hpwxx@ast-mbp.dhcp.thefacebook.com>
Date:   Thu, 18 Oct 2018 22:00:53 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Nadav Amit <namit@...are.com>, Oleg Nesterov <oleg@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        "Woodhouse, David" <dwmw@...zon.co.uk>, daniel@...earbox.net,
        guro@...com
Subject: Re: [RFC PATCH 1/5] x86: introduce preemption disable prefix

> 
> >
> > Another example is __BPF_PROG_RUN_ARRAY(), which also uses
> > preempt_enable_no_resched().
> 
> Alexei, I think this code is just wrong.

why 'just wrong' ?

> Do you know why it uses
> preempt_enable_no_resched()?

dont recall precisely.
we could be preemptable at the point where macro is called.
I think the goal of no_resched was to avoid adding scheduling points
where they didn't exist before just because a prog ran for few nsec.
May be Daniel or Roman remember.