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vgacon.c: add cond reschedule points in vgacon_do_font_op

PATCH In review Marcelo Tosatti 2 participants 2 messages Jun 20, 2013 – Jun 26, 2013

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Marcelo Tosatti

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#0 lore raw

PATCH 1/1 vgacon.c: add cond reschedule points in vgacon_do_font_op

Booting a 64-vcpu KVM guest, with CONFIG_PREEMPT_VOLUNTARY, can result in a soft lockup:

BUG: soft lockup - CPU#41 stuck for 67s! [setfont:1505]
 RIP: 0010:[<ffffffff812c48da>]
[<ffffffff812c48da>] vgacon_do_font_op.clone.0+0x1ba/0x550

This is due to the 8192 (cmapsz) IO operations taking longer than expected due to lock contention in QEMU.

Add conditional resched points in between writes allowing other tasks to execute.

Signed-off-by Marcelo Tosatti <[email protected]>
drivers/video/console/vgacon.c +12 -4
index 5855d17..0dd6d96 100644
@@ -1124,11 +1124,15 @@ static int vgacon_do_font_op(struct vgastate *state,char *arg,int set,int ch512)
 
 	if (arg) {
 		if (set)
-			for (i = 0; i < cmapsz; i++)
+			for (i = 0; i < cmapsz; i++) {
 				vga_writeb(arg[i], charmap + i);
+				cond_resched();
+			}
 		else
-			for (i = 0; i < cmapsz; i++)
+			for (i = 0; i < cmapsz; i++) {
 				arg[i] = vga_readb(charmap + i);
+				cond_resched();
+			}
 
 		/*
 		 * In 512-character mode, the character map is not contiguous if

@@ -1139,11 +1143,15 @@ static int vgacon_do_font_op(struct vgastate *state,char *arg,int set,int ch512)
 			charmap += 2 * cmapsz;
 			arg += cmapsz;
 			if (set)
-				for (i = 0; i < cmapsz; i++)
+				for (i = 0; i < cmapsz; i++) {
 					vga_writeb(arg[i], charmap + i);
+					cond_resched();
+				}
 			else
-				for (i = 0; i < cmapsz; i++)
+				for (i = 0; i < cmapsz; i++) {
 					arg[i] = vga_readb(charmap + i);
+					cond_resched();
+				}
 		}
 	}
#1 lore raw
Quoted 11 lines

can result in a soft lockup:

BUG: soft lockup - CPU#41 stuck for 67s! [setfont:1505]
 RIP: 0010:[<ffffffff812c48da>]
[<ffffffff812c48da>] vgacon_do_font_op.clone.0+0x1ba/0x550

This is due to the 8192 (cmapsz) IO operations taking longer than expected due to lock contention in QEMU.

Add conditional resched points in between writes allowing other tasks to execute.

I've applied this to drm-next against my best judgement as it steers me close to being the console maintainer.

Dave.