linux-kernel - Re: [PATCH] char_dev: replace cdev

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Date:   Mon, 11 Jan 2021 18:51:11 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Christoph Hellwig <hch@....de>, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] char_dev: replace cdev_map with an xarray

On Mon, Jan 11, 2021 at 07:33:15PM +0100, Greg KH wrote:
> On Mon, Jan 11, 2021 at 06:20:29PM +0000, Matthew Wilcox wrote:
> > > efficient in what way?  Space or faster lookup?
> > 
> > Both, but primarily space.
> > 
> > The radix tree underlying the xarray allows N consecutive entries with
> > the same value to be represented as a single entry; if there are at
> > least 64 entries then we get to skip an entire level of the tree (saving
> > 1/7 of a page).  Of course, we'd need to go from the 'head' pointer to
> > the correct pointer, something like p += rdev - p->rdev.
> 
> How much "space" are you talking about here?

576 bytes -- 1/7 of a page.

> A "normal" machine has about 100-200 char devices.  Servers, maybe more,
> but probably not.
> 
> The kobject being used previously wasn't really "small" at all, so odds
> are any conversion to not use it like this will be better overall.

Yes.

> > > THis shouldn't be on a "fast" lookup path, so I doubt that's worth
> > > optimizing for.  Space, maybe, for systems with thousands of scsi
> > > devices, but usually they just stick to the block device, not a char
> > > device from what I remember.
> > 
> > /dev/sgX is a chardev?
> 
> I sure hope no one is using /dev/sgX for tens of thousands of block
> device accesses, if so, they have bigger problems than this :)

There is one sgX char dev for every /dev/sdN, so anyone with a thousand
SCSI devices also has a thousand char devices.  On the other hand,
they're added one at a time, so there is no chance to optimise here:

        cdev = cdev_alloc();
...
        error = cdev_add(cdev, MKDEV(SCSI_GENERIC_MAJOR, sdp->index), 1);