Hi Sam Am 10.02.22 um 22:16 schrieb Sam Ravnborg:
Hi Thomas, On Thu, Feb 10, 2022 at 03:11:13PM +0100, Thomas Zimmermann wrote:Fbdev's deferred I/O sorts all dirty pages by default, which incurs a significant overhead. Make the sorting step optional and update the few drivers that require it. Use a FIFO list by default. Sorting pages by memory offset for deferred I/O performs an implicit bubble-sort step on the list of dirty pages. The algorithm goes through the list of dirty pages and inserts each new page according to its index field. Even worse, list traversal always starts at the first entry. As video memory is most likely updated scanline by scanline, the algorithm traverses through the complete list for each updated page. For example, with 1024x768x32bpp a page covers exactly one scanline. Writing a single screen update from top to bottom requires updating 768 pages. With an average list length of 384 entries, a screen update creates (768 * 384 =) 294912 compare operation. Fix this by making the sorting step opt-in and update the few drivers that require it. All other drivers work with unsorted page lists. Pages are appended to the list. Therefore, in the common case of writing the framebuffer top to bottom, pages are still sorted by offset, which may have a positive effect on performance. Playing a video [1] in mplayer's benchmark mode shows the difference (i7-4790, FullHD, simpledrm, kernel with debugging). mplayer -benchmark -nosound -vo fbdev ./big_buck_bunny_720p_stereo.ogg With sorted page lists: BENCHMARKs: VC: 32.960s VO: 73.068s A: 0.000s Sys: 2.413s = 108.441s BENCHMARK%: VC: 30.3947% VO: 67.3802% A: 0.0000% Sys: 2.2251% = 100.0000% With unsorted page lists: BENCHMARKs: VC: 31.005s VO: 42.889s A: 0.000s Sys: 2.256s = 76.150s BENCHMARK%: VC: 40.7156% VO: 56.3219% A: 0.0000% Sys: 2.9625% = 100.0000% VC shows the overhead of video decoding, VO shows the overhead of the video output. Using unsorted page lists reduces the benchmark's run time by ~32s/~25%.Nice!Signed-off-by: Thomas Zimmermann <tzimmermann@xxxxxxx> Link: https://download.blender.org/peach/bigbuckbunny_movies/big_buck_bunny_720p_stereo.ogg # [1] --- drivers/staging/fbtft/fbtft-core.c | 1 + drivers/video/fbdev/broadsheetfb.c | 1 + drivers/video/fbdev/core/fb_defio.c | 19 ++++++++++++------- drivers/video/fbdev/metronomefb.c | 1 + drivers/video/fbdev/udlfb.c | 1 + include/linux/fb.h | 1 + 6 files changed, 17 insertions(+), 7 deletions(-) diff --git a/drivers/staging/fbtft/fbtft-core.c b/drivers/staging/fbtft/fbtft-core.c index f2684d2d6851..4a35347b3020 100644 --- a/drivers/staging/fbtft/fbtft-core.c +++ b/drivers/staging/fbtft/fbtft-core.c @@ -654,6 +654,7 @@ struct fb_info *fbtft_framebuffer_alloc(struct fbtft_display *display, fbops->fb_blank = fbtft_fb_blank;fbdefio->delay = HZ / fps;+ fbdefio->sort_pagelist = true; fbdefio->deferred_io = fbtft_deferred_io; fb_deferred_io_init(info);diff --git a/drivers/video/fbdev/broadsheetfb.c b/drivers/video/fbdev/broadsheetfb.cindex fd66f4d4a621..b9054f658838 100644 --- a/drivers/video/fbdev/broadsheetfb.c +++ b/drivers/video/fbdev/broadsheetfb.c @@ -1059,6 +1059,7 @@ static const struct fb_ops broadsheetfb_ops = {static struct fb_deferred_io broadsheetfb_defio = {.delay = HZ/4, + .sort_pagelist = true, .deferred_io = broadsheetfb_dpy_deferred_io, };diff --git a/drivers/video/fbdev/core/fb_defio.c b/drivers/video/fbdev/core/fb_defio.cindex 3727b1ca87b1..1f672cf253b2 100644 --- a/drivers/video/fbdev/core/fb_defio.c +++ b/drivers/video/fbdev/core/fb_defio.c @@ -132,15 +132,20 @@ static vm_fault_t fb_deferred_io_mkwrite(struct vm_fault *vmf) if (!list_empty(&page->lru)) goto page_already_added;- /* we loop through the pagelist before adding in order- to keep the pagelist sorted */ - list_for_each_entry(cur, &fbdefio->pagelist, lru) { - if (cur->index > page->index) - break; + if (fbdefio->sort_pagelist) { + /* + * We loop through the pagelist before adding in order + * to keep the pagelist sorted. + */ + list_for_each_entry(cur, &fbdefio->pagelist, lru) { + if (cur->index > page->index) + break; + } + list_add_tail(&page->lru, &cur->lru); + } else { + list_add_tail(&page->lru, &fbdefio->pagelist); }Bikeshedding - my personal style is to have the likely part first. This makes reading the code easier.
I'll change this a bit to leave out the else branch.
The following drivers uses deferred io but are not listed as they need the page list sorted: - hecubafb - hyperv_fb - sh_mobile_lcdcfb - smscufx - ssd1307fb - xen-fbfront It would be nice with some info in the commit log that they do not need the pages sorted. To make the list complete include the drm stuff too. It did not jump to me why they did not need sorted pages, so some sort of reassurance that they have been checked would be nice.
Most drivers build a bounding rectangle around the dirty pages or simply flush the whole screen. The only two affected DRM drivers, generic fbdev and vmwgfx, both use the bounding rectangle. In those cases, the exact order of the pages doesn't matter. The other drivers look at the page index or handle pages one-by-one. I set the sort_pagelist flag for those, even though some of them would probably work correctly without sorting.
I'll add this information to the commit description. Best regards Thomas
With the following addressed: Acked-by: Sam Ravnborg <sam@xxxxxxxxxxxx> I hope someone else looks that can verify that the list of drivers without sort_pagelist is correct so someone knowledgeable have looked too. Sam
-- Thomas Zimmermann Graphics Driver Developer SUSE Software Solutions Germany GmbH Maxfeldstr. 5, 90409 Nürnberg, Germany (HRB 36809, AG Nürnberg) Geschäftsführer: Ivo Totev
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