linux-kernel - Re: [PATCH v2 7/9] sched: define TIF_ALLOW

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Date:   Sun, 24 Sep 2023 08:19:04 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ankur Arora <ankur.a.arora@...cle.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org, x86@...nel.org,
        akpm@...ux-foundation.org, luto@...nel.org, bp@...en8.de,
        dave.hansen@...ux.intel.com, hpa@...or.com, mingo@...hat.com,
        juri.lelli@...hat.com, vincent.guittot@...aro.org, mgorman@...e.de,
        rostedt@...dmis.org, jon.grimm@....com, bharata@....com,
        raghavendra.kt@....com, boris.ostrovsky@...cle.com,
        konrad.wilk@...cle.com, jgross@...e.com, andrew.cooper3@...rix.com
Subject: Re: [PATCH v2 7/9] sched: define TIF_ALLOW_RESCHED

On Sun, Sep 24, 2023 at 12:50:43AM +0200, Thomas Gleixner wrote:
> cond_resched() cannot nest and is obviously scope-less.
> 
> The TIF_ALLOW_RESCHED mechanism, which sparked this discussion only
> pretends to be scoped.
> 
> As Peter pointed out it does not properly nest with other mechanisms and
> it cannot even nest in itself because it is boolean.

We can nest a single bit without turning it into a counter -- we
do this for memalloc_nofs_save() for example.  Simply return the
current value of the bit, and pass it to _restore().

eg xfs_prepare_ioend():

        /*
         * We can allocate memory here while doing writeback on behalf of
         * memory reclaim.  To avoid memory allocation deadlocks set the
         * task-wide nofs context for the following operations.
         */
        nofs_flag = memalloc_nofs_save();

        /* Convert CoW extents to regular */
        if (!status && (ioend->io_flags & IOMAP_F_SHARED)) {
                status = xfs_reflink_convert_cow(XFS_I(ioend->io_inode),
                                ioend->io_offset, ioend->io_size);
        }

        memalloc_nofs_restore(nofs_flag);

I like your other approach better, but just in case anybody starts
worrying about turning a bit into a counter, there's no need to do
that.